Create a script to execute msys2 and maintain PATH and PKG_CONFIG_PATH

@eyal0 you might want to extend this Action with those two lines, in order to make them easier to maintain and reuse. I would suggest the following:

1. Fork this repo.
2. Create a feature branch.
3. Locally, execute npm run install. Either get NodeJs, use a node container, or use WSL.
4. Edit https://github.com/msys2/setup-msys2/blob/master/main.js#L163-L174, so that an additional file is written, which contains your custom code.
5. Execute npm run pkg.
6. Add index.js, commit and push.
7. Edit your workflow for using eyal0/setup-msys2@feature-branch-name and change the shell: yourcustomentrypoint {0}. Commit and push.
8. Go back to 4-6, and restart the worflow until you get it done.

_Originally posted by @eine in https://github.com/msys2/setup-msys2/issues/98#issuecomment-757538531_

[eyal0]

I found the cause of those extra lines and filed a bug. I think that it would be fixed pretty easily.

[eine]

See the continuation of this issue in #171.