There is no active maintainer for cygwin.  The CI failure is too noisy.

[Misc #16407]

`shell: bash` runs bash on msys which prefers git on msys too,
then checked out in CRLF mode.  Cygwin sed doesn't consider the CR
a part of EOL code, though.

mame-san said it's weird

Since 8c9450e7b875db846b19cc631af0d7fee66db5c6, we increased the chance
to run GitHub Actions on a fork, as we usually use a topic branch
instead of master when filing a pull request.

This patch makes it possible to reuse the same GitHub Actions config on
a fork repository.