Source devnull in environment_after_sourcing_files (closes #15775) (#15791)

spack.util.environment_after_sourcing_files compares the local
environment against a shell environment after having sourced a
file; but this ends up including the default shell profile and
rc, which might differ from the local environment.

To change this, compare against the default shell environment,
expressed here as 'source /dev/null'.

lib/spack/spack/util/environment.py

@@ -604,7 +604,8 @@ def from_sourcing_file(filename, *arguments, **kwargs):
 
         # Compute the environments before and after sourcing
         before = sanitize(
-            dict(os.environ), blacklist=blacklist, whitelist=whitelist
+            environment_after_sourcing_files(os.devnull, **kwargs),
+            blacklist=blacklist, whitelist=whitelist
         )
         file_and_args = (filename,) + arguments
         after = sanitize(