Ensure flake is locked at start of CI run

[purcell]

What do you think of this, @akirak?

Maybe it's more trouble than it's worth, but it wasn't obvious how we could automatically verify that the flake.lock is not out of sync.

[akirak]

@purcell I think it's fine to update the lock file this way. Sorry for the previous mistake.

Alternatively, we could check if nix profile install works with the same options as setup-emacs? That's what we want to test.

It's confusing that nix profile install and nix run/nix build handle an out-of-sync lock file in different ways. In the future, Nix may change how it handles these situations, as Nix flakes are currently not completely stable (it's still experimental). The test should be as close as possible to the major use case of nix-emacs-ci (i.e. setup-emacs).

I also noticed that tests/default.nix can be converted to Emacs Lisp code. It is possible to first install Emacs using nix profile install and then test it in a classic Emacs way without the Nix shell application.

[purcell]

I also noticed that tests/default.nix can be converted to Emacs Lisp code

Maybe, but that file is used by the flake checks: https://github.com/purcell/nix-emacs-ci/blob/master/flake.nix#L163

and those are what the CI runs.

It's confusing that nix profile install and nix run/nix build handle an out-of-sync lock file in different ways.

Yes, that's the root problem I guess.

[purcell]

Sorry for the previous mistake.

Also, no apology is necessary! We learned something useful.

[akirak]

It's nice to ensure flake.lock is in sync.

Alternatively, we could check if nix profile install works with the same options as setup-emacs?

This is a separate problem, so we can merge this PR anyway.