Consider logging installed version of Nix

[marienz]

nix-installer-action downloads an unversioned nix-installer (#84), which installs (I think, as of DeterminateSystems/nix-installer#897) an embedded version of Nix.

I can't work out from the Github Action (pretty or raw) logs which version of either of these components the action installed. It seems to be grabbing https://install.determinate.systems/nix-installer/stable/x86_64-linux from a cache instead of downloading it, I don't see a version number in the filename invoked, and neither nix-installer nor nix seem to log their version by default. Raw logs in case I missed something: logs.txt

This adds some annoyance to troubleshooting failing CI, both because the version may change underneath me and because I need to know the Nix version to reproduce elsewhere.

I can add an action that logs some info about Nix, but I think it would be convenient if nix-installer-action provided this information by default. I'm more interested in the version of Nix than the version of the installer.

This isn't the same as #84: I don't necessarily want to pin the version of Nix (or nix-installer), but I do want to know which version any given build ended up using.