Put cygwin filemode tests back under the sourcedir
Remove inheritable permissions from the sourcedir
For :reasons:, the unit tests which check file mode were built in the
tempdir.
Instead, remove inheritable permissions from the working directory
(which the GitHub VM image has set for some reaons), since they can
interfere with getting exactly the file mode you asked for.
Partially reverts 04ae1cfb79
(cherry picked from commit f164ec692f)
ae6fbc3ccb