Previously the code would not chmod for the Windows hermetic toolchains
because there is usually no need - Windows does not have chmod and
if you are downloading the Windows repo on a UNIX system, you won't
run it, so it will stay as is.

However, that left a single case where somebody may want to download
the Linux toolchain on a Windows and the main cases are:
* `bazel sync`
* build a docker image on Windows using `rules_oci` or similar.

Fixes bazel-contrib#2660