Debugging support: fix compiler wrapper log on Mac OS (#20333)

This fixes a logging error observed on macOS 11.0.1 (Big Sur).
When performing a Spack install in debugging mode (e.g.
`spack -d install py-scipy`) Spack is supposed to write a log of
compiler wrapper command line invocations to the current working
directory.

Due to a regression error introduced by #18205, these files were
no-longer generated, and Spack was printing errors such as
"No such file or directory: None/." This is because the log file
directory gets set from `spack.main.spack_working_dir`, but that
variable is not set in the spawned process.

This PR ensures that the working directory (at the time of the
"spack install" invocation) is persisted to the subprocess.

lib/spack/spack/subprocess_context.py

@@ -69,10 +69,12 @@ def __init__(self, pkg):
             self.serialized_pkg = serialize(pkg)
         else:
             self.pkg = pkg
+        self.spack_working_dir = spack.main.spack_working_dir
         self.test_state = TestState()
 
     def restore(self):
         self.test_state.restore()
+        spack.main.spack_working_dir = self.spack_working_dir
         if _serialize:
             return pickle.load(self.serialized_pkg)
         else: