Supply multiple mirrors for Cygwin to download from

[stertooy]

As suggested by @Joseph-Edwards in gap-system/gap#6233 (comment)

This doesn't work perfectly, though, as the Cygwin installer will not always switch to the second mirror if the first one fails. In particular, it is not configured to switch mirrors when it encounters connection error: 12029, which is exactly the error that occured after the recent hardware failure at mirrors.kernel.org.

Maybe a more robust solution would be something like

    - name: "Download and install Cygwin (Mirror 1)"
      uses: cygwin/cygwin-install-action@v6
      continue-on-error: true
      with:
        packages: "${{ inputs.pkgs-to-install}},${{ inputs.extra-pkgs-to-install }}"
        add-to-path: false
        install-dir: C:\cygwin64
        site: [ mirror 1 ]
        
    - name: "Download and install Cygwin (Mirror 2)"
      uses: cygwin/cygwin-install-action@v6
      if: [ only run if previous step failed ]
      with:
        packages: "${{ inputs.pkgs-to-install}},${{ inputs.extra-pkgs-to-install }}"
        add-to-path: false
        install-dir: C:\cygwin64
        site: [ mirror 2 ]

But that does make the action more messy...

[Joseph-Edwards]

In the first instance, I think this is a good change that adds a backup without adding complexity.

If the connection_error: 12029 is something that starts to happen more frequently, then I think the "more robust" fix would be sensible. Since it seems like a one-off (hopefully), I think the proposed change is sufficient for now.

[fingolfin]

Thanks! Someone else feel free to (re)tag, I'm afk