[LLD] Fix probing a MSYS based 'tar' in a Windows Container

[mstorsjo]

Summary

Don't run the 'tar' tool in a cleared environment with only the
LANG variable set, just set LANG on top of the existing environment.

If the 'tar' tool is an MSYS based tool, running it in a Windows
Container hangs if all environment variables are cleared - in
particular, the USERPROFILE variable needs to be kept intact.

This is the same issue fixed as was fixed in other places in
9de63b2e051cb3e79645cc20b83b4d33d132cba0, but contrary to running
the actual tests, running with an as-cleared-as-possible environment
here is less important.

[rnk]

lgtm

lld/test/lit.cfg.py

 if config.have_dia_sdk:
     config.available_features.add("diasdk")
 
 if config.sizeof_void_p == 8:
     config.available_features.add("llvm-64-bits")
 
 tar_executable = lit.util.which('tar', config.environment['PATH'])
 if tar_executable:
+    env = os.environ
+    env['LANG'] = 'C'
     tar_version = subprocess.Popen(
         [tar_executable, '--version'],
         stdout=subprocess.PIPE,
         stderr=subprocess.PIPE,
-        env={'LANG': 'C'})
+        env=env)
     sout, _ = tar_version.communicate()
     if 'GNU tar' in sout.decode():
         config.available_features.add('gnutar')
 
 # ELF tests expect the default target for ld.lld to be ELF.
 if config.ld_lld_default_mingw:
     config.excludes.append('ELF')