Don't puke in the release workflow if the Nix version didn't change (#1461)

D'oh, `git commit` doesn't like it if you have no diff.

Author: grahamc

.github/workflows/propose-release.yml

@@ -37,4 +37,4 @@ jobs:
 
         sed -i 's#https://flakehub.com/f/DeterminateSystems/nix/=.*";#https://flakehub.com/f/DeterminateSystems/nix/=${{ inputs.nix-version }}";#' ./flake.nix
         git add flake.nix
-        git commit -m "Update Nix release to ${{ inputs.nix-version }}"
+        git commit -m "Update Nix release to ${{ inputs.nix-version }}" || true