environment_modifications_for_specs: do not mutate spec.prefix (#41737)

Sometimes env variables computed in `setup_run_environment` depend on tests
w.r.t. files in `spec.prefix`, but Spack temporarily projects `spec.prefix` to
the view. 

This is problematic for two reasons:

1. Some packages iterate over `<prefix>/bin`: they expect only the current
   package's executables, but find all linked in the view, leading to false
   positives.
2. Some packages test for `os.path.islink(...)`, which is always true in a view

`gcc` is an example that does both.

This PR lets Spack compute the environment modifications using the original
prefix, and projects to the view afterwards

0 files changed, 0 insertions, 0 deletions