[ci] Add cygwin bin directories to PATH

[tobil4sk]

Without these PATH entries, cygwin's binaries never get used. This means that make and bash and other executables are not used from cygwin, but from other preinstalled packages in the github actions runner (e.g. git bash and the preinstalled mingw).

This causes build issues with certain packages, see:
#26501
aantron/luv#162

[dra27]

Something else is going on here - it shouldn’t be necessary to do this in a package build - I’ll endeavour to have a look tomorrow/next week

[tobil4sk]

This seems somewhat related to ocaml/opam#6181. To me the ideal solution seems to be for opam env/opam exec to take care of setting the PATH correctly so that it is not necessary to set the PATH manually.

[tobil4sk]

Should mingw-w64-shims also add /usr/x86_64-w64-mingw32/bin to PATH maybe? It already adds /usr/x86_64-w64-mingw32/sys-root/mingw/bin: https://github.com/dra27/mingw-w64-shims/blob/3c4a82700d9b03443ad6f2927449f7a8f25a77cf/mingw-w64-shims.opam#L8

The problem is that the luv library builds a native library using autotools, and the configure script expects an ar executable to be available in PATH. Since /usr/x86_64-w64-mingw32/bin isn't present in PATH, the build fails (both in CI and on local systems where the PATH hasn't been configured like this).

Does adding this to mingw-w64-shims sound like a reasonable idea? If so, I could try to send a patch there instead and close this PR.

EDIT: I've gone and opened: dra27/mingw-w64-shims#3