workflow: expose runs-on as an input

[colemickens]

testing in progress.

I need this to coerce the inventory to eval on a beefier machine than ubuntu-latest.

I think this exposes a slight awkwardness of reusable workflows. Not sure if there's a more elegant way, just want to send this before I forget, it's getting late 😆 .

[colemickens]

Initial test looks like this might not work. Again, mostly note for self for when I or someone else returns to this.

[colemickens]

Really great job there GitHub. Let's try again:

[lucperkins]

FYI the syntax is generally ${{ ... }}. I imagine it's tolerating this syntax but worth being consistent.