Add ImageOS companion to ImageVersion environment variable

This request is suggested as a means of simplifying the inputs needed to some actions.

In, new images are tagged as below:


ImageVersion is the second part of the tag.  Having the first part of the tag (‘OS String’) set to ImageOS would be helpful.  As to macOS, maybe just that?

One use is building separate packages/binaries for use on all platforms.  Obviously, one can use ‘runs-on:’ or ‘${{ matrix.os }}’ as an action input, but having the image name as an env variable would be simpler…

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