config: fix `SPACK_DISABLE_LOCAL_CONFIG`, remove `$user_config_path` (#27022)

There were some loose ends left in ##26735 that cause errors when
using `SPACK_DISABLE_LOCAL_CONFIG`.

- [x] Fix hard-coded `~/.spack` references in `install_test.py` and `monitor.py`

Also, if `SPACK_DISABLE_LOCAL_CONFIG` is used, there is the issue that
`$user_config_path`, when used in configuration files, makes no sense,
because there is no user config scope.

Since we already have `$user_cache_path` in configuration files, and since there
really shouldn't be *any* data stored in a configuration scope (which is what
you'd configure in `config.yaml`/`bootstrap.yaml`/etc., this just removes
`$user_config_path`.

There will *always* be a `$user_cache_path`, as Spack needs to write files, but
we shouldn't rely on the existence of a particular configuration scope in the
Spack code, as scopes are configurable, both in number and location.

- [x] Remove `$user_config_path` substitution.
- [x] Fix reference to `$user_config_path` in `etc/spack/deaults/bootstrap.yaml`
      to refer to `$user_cache_path`, which is where it was intended to be.

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