Tag deployments with some useful identifier (GNOME version?)

[ferdnyc]

While I certainly hope that backports to older GNOME versions won't be a super-common thing, to make our deployments usable for future backport testing we should probably be tagging them with some sort of semi-permanent identifier, in addition to the latest always being tagged main.

The GNOME version incorporated into the test environment seems like a natural fit, and would likely be the parameter by which we'd want to select older deployments in the rare cases that they're needed again.

[ferdnyc]

@andyholmes noticed this a while back, too.