workflows: instead of opening a PR, directly commit

Signed-off-by: John Titor <50095635+JohnRTitor@users.noreply.github.com>

Author: JohnRTitor

.github/workflows/build-and-release.yml

@@ -1,6 +1,6 @@
 name: Release NixOS unstable ISO
 concurrency:
-  group: build-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
 on:

.github/workflows/update-flake.yml

@@ -1,6 +1,6 @@
 name: update-flake-lock
 concurrency:
-  group: build-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
 on:
@@ -21,9 +21,14 @@ jobs:
         uses: cachix/install-nix-action@v25
         with:
           extra_nix_config: |
+            auto-optimise-store = true
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+            experimental-features = nix-command flakes
             accept-flake-config = true
-      - name: Reformat *.nix files
-        run: nix fmt
       - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v21
+        run: nix flake update
+      - name: Commit changes
+        uses: EndBug/add-and-commit@v9
+        with:
+          default_author: github_actions
+          add: flake.lock