simplify changing cachix cache for install tests

[abathur]

@domenkozar In the process of getting set up to try out the new installer tests that landed in #4549 I noticed a couple of small workflow improvements:

• Make cachix cache name deterministic to avoid workflow friction (having to re-make this change every time, remember to take it back out before merge or test it on a separate branch, etc.)
  • I wanted to make this a secret to make it easily configurable, but GA apparently won't allow an output (i.e., the installerURL) to have a secret in it.
  • Instead I'm using lowercase<username>-<repo>-install-tests (i.e., abathur-nix-install-tests)
• Set CACHIX_AUTH_TOKEN in addition to CACHIX_SIGNING_KEY so that both methods are supported. It looks like cachix will try signing key first, then auth token.

Test runs in my fork for this change:

• successful install tests
• skip install tests if neither secret is declared (implying the user isn't working on the installer?)

If adopted, Nix would need a new nixos-nix-install-tests cache.

[abathur]

@domenkozar I've modified this after the initial plan didn't quite work out.

I wanted to just move the CACHIX_NAME into the repo secrets so that it'd be easy to set once per fork, but I didn't realize GA would literally prevent it from appearing in the output of a step (which breaks the installerURL).

I still think there's ergonomic value in something lower-friction than having to modify the workflow, so I've fallen back on a deterministic CACHIX_NAME (lowercase of <username>-<repo>-install-tests) so the user can just create the matching cache and add a token/key for it to the repo's secrets. I also figured out how to keep these from trying to run if the secret isn't set. (links to runs in OP)

[domenkozar]

Seems to work! https://github.com/NixOS/nix/runs/2087652478

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/anyone-up-for-picking-at-some-nix-onboarding-improvements/13152/1

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/installer-test-suite-small-project-s-high-leverage-help-wanted/13662/1