config: make path replacements lazy (#34758)

Currently, all of the replacements in `spack.util.path.replacements()` get evaluated for
each replacement. This makes it easy to get bootstrap issues, because config is used
very early on in Spack.

Right now, if I run `test_autotools_gnuconfig_replacement_no_gnuconfig` on my M1 mac, I
get the circular reference error below. This fixes the issue by making all of the path
replacements lazy lambdas.

As a bonus, this cleans up the way we do substitution for `$env` -- it's consistent with
other substitutions now.

- [x] make all path `replacements()` lazy
- [x] clean up handling of `$env`

```console
> spack unit-test -k test_autotools_gnuconfig_replacement_no_gnuconfig

...

==> [2022-12-31-15:44:21.771459] Error: AttributeError:

The 'autotools-config-replacement' package cannot find an attribute while trying to build from sources. This might be due to a change in Spack's package format to support multiple build-systems for a single package. You can fix this by updating the build recipe, and you can also report the issue as a bug. More information at https://spack.readthedocs.io/en/latest/packaging_guide.html#installation-procedure

/Users/gamblin2/src/spack/lib/spack/spack/package_base.py:1332, in prefix:
       1330    @property
       1331    def prefix(self):
  >>   1332        """Get the prefix into which this package should be installed."""
       1333        return self.spec.prefix

Traceback (most recent call last):
  File "/Users/gamblin2/src/spack/lib/spack/spack/build_environment.py", line 1030, in _setup_pkg_and_run
    kwargs["env_modifications"] = setup_package(
                                  ^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/build_environment.py", line 757, in setup_package
    set_module_variables_for_package(pkg)
  File "/Users/gamblin2/src/spack/lib/spack/spack/build_environment.py", line 596, in set_module_variables_for_package
    m.std_cmake_args = spack.build_systems.cmake.CMakeBuilder.std_args(pkg)
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/build_systems/cmake.py", line 241, in std_args
    define("CMAKE_INSTALL_PREFIX", pkg.prefix),
                                   ^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/package_base.py", line 1333, in prefix
    return self.spec.prefix
           ^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/spec.py", line 1710, in prefix
    self.prefix = spack.store.layout.path_for_spec(self)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/directory_layout.py", line 336, in path_for_spec
    path = self.relative_path_for_spec(spec)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/directory_layout.py", line 106, in relative_path_for_spec
    projection = spack.projections.get_projection(self.projections, spec)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/projections.py", line 13, in get_projection
    if spec.satisfies(spec_like, strict=True):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/spec.py", line 3642, in satisfies
    if not self.virtual and other.virtual:
           ^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/spec.py", line 1622, in virtual
    return spack.repo.path.is_virtual(self.name)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/repo.py", line 890, in is_virtual
    return have_name and pkg_name in self.provider_index
                                     ^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/repo.py", line 770, in provider_index
    self._provider_index.merge(repo.provider_index)
                               ^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/repo.py", line 1096, in provider_index
    return self.index["providers"]
           ~~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/repo.py", line 592, in __getitem__
    self._build_all_indexes()
  File "/Users/gamblin2/src/spack/lib/spack/spack/repo.py", line 607, in _build_all_indexes
    self.indexes[name] = self._build_index(name, indexer)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/spack/repo.py", line 616, in _build_index
    index_mtime = self.cache.mtime(cache_filename)
                  ^^^^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/llnl/util/lang.py", line 826, in __getattr__
    return getattr(self.instance, name)
                   ^^^^^^^^^^^^^
  File "/Users/gamblin2/src/spack/lib/spack/llnl/util/lang.py", line 825, in __getattr__
    raise AttributeError()
AttributeError
```

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