Require flake.lock to be up-to-date if it exists

[cole-h]

This will prevent uploading of flakes that need to be re-locked.

[cole-h]

Tested with:

• No flake.lock: still works
• With up-to-date flake.lock: still works
• With out-of-date flake.lock: no longer works

(Looks similar to below)

Error:
   0: Checking flake evaluates
   1: Failed to execute command `nix flake show --all-systems --json --no-update-lock-file /home/vin/workspace/detsys/flakehub/docs-test-thing/test-repo/` with status 1
      stdout:
      stderr: warning: Git tree '/home/vin/workspace/detsys/flakehub/docs-test-thing/test-repo' is dirty
      error:
             … while updating the lock file of flake 'git+file:///home/vin/workspace/detsys/flakehub/docs-test-thing/test-repo'

             error: flake 'git+file:///home/vin/workspace/detsys/flakehub/docs-test-thing/test-repo' requires lock file changes but they're not allowed due to '--no-update-lock-file'

   1:

Location:
   src/flake_info.rs:96

stale

[cole-h]

This is the special sauce right here: if nix flake metadata --json would have updated the flake.lock, this causes Nix (and thus the entire upload) to fail (which we want).

[colemickens]

super nit: constuct vec of args, use (Command).args() to add it to the command, then you can re-use the args for the error message instead of repeating it. (Just in case we change the command again later, so we don't to change error strings too). But this is mostly just golfing.

but LGTM