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
ec2729706b