Bootstrap GnuPG and file on Windows (#41810)

Spack can now bootstrap two new dependencies on Windows: GnuPG, and file. 

These dependencies are modeled as a separate package, and they install a cross-compiled binary.
Details on how they binaries are built are in https://github.com/spack/windows-bootstrap-resources

share/spack/qa/bootstrap-file.py

@@ -0,0 +1,4 @@
+from spack.util.filesystem import file_command
+
+if __name__ == "__main__":
+    file_command()