Allow keeping Cygwin out of PATH

[me-and]

In some circumstances, it might be useful to have Cygwin installed, but not have its executables take precedence over pre-installed packages. Add an "add-to-path" option that, if set to "false", will skip the step of adding Cygwin's /bin directory to PATH.

Also update documentation to mention this option, and update the test scripts to check it works as expected; the default behaviour is tested as part of the "install-dir-test", and the new behaviour as part of the main test.

My specific use case is with actions/cache: the only way I've been able to get that to work is by making sure that Cygwin's binaries aren't in the system path. Without that, I get the following error when the action attempts to store something in the cache:

Post job cleanup.
"C:\Program Files\Git\usr\bin\tar.exe" --posix -cf cache.tgz --exclude cache.tgz -P -C D:/a/Cygwin-GitHub-Actions-playground/Cygwin-GitHub-Actions-playground --files-from manifest.txt --force-local -z
/usr/bin/tar: cache.tgz: Cannot write: Broken pipe
/usr/bin/tar: Child returned status 1[2](https://github.com/me-and/Cygwin-GitHub-Actions-playground/actions/runs/3771792811/jobs/6412370832#step:19:2)7
/usr/bin/tar: Error is not recoverable: exiting now
Warning: Failed to save: "C:\Program failed with error: The process 'C:\Program Files\Git\usr\bin\tar.exe' failed with exit code 2

I'm sure there's a more "correct" solution here, but providing the option of not putting Cygwin in the PATH seems like it could be useful more generally, and I've confirmed it solves my immediate problem.

[jon-turney]

I'm sure there's a more "correct" solution here,

Yes. This also seems to cause problems with actions/checkout@v3's post-run step when cygwin git is in the path. The workaround I ended up with just doing was 'rm /usr/bin/git' before post-run steps started.

If we could register a post-run step to unwind the path change, that would be best, but I'm not sure if that can be done at all (only adding to the path seems allowed), and it seems that composite actions don't support post-run steps, so the whole thing would have to be re-written as a JS or docker action.

Since this seems generally useful anyhow, I merged it :)