webfactory/ssh-agent is not working with setup-msys2

[mojca]

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I described the problem in https://github.com/webfactory/ssh-agent/issues/149 and I'm not actually sure which action needs to be fixed, but it would certainly be helpful to collaborate to find a suitable solution.

The webfactory/ssh-agent action allows specifying a private SSH key to fetch dependencies via CMake's FetchContent functionality. It seems to work successfully on all major platforms: Linux, macOS and Windows, but I'm not sure how to make it work from within the msys2 shell.

[eine]

When using SSH (or git) on MSYS2, private keys are searched for at ~/.ssh/, which defaults to <path-to-msys2-installation-drive-or-path>/msys64/home/<user>/.ssh. Therefore, I guess that webfactory/ssh-agent needs to accept some option to put the private keys there, instead of somewhere else.