upload flake diff

.github/workflows/flake-update.yml

@@ -47,11 +47,15 @@ jobs:
               run: nix ./utils/eval-to-drv.sh post
             - name: Calculate diff
               run: nix ./utils/diff-evals.sh
-            - name: Read file contents
-              id: read_file
-              uses: guibranco/github-file-reader-action-v2@latest
+            - name: upload diff file as artifact
+              id: upload-diff
+            - uses: actions/upload-artifact@v3
               with:
-                path: "post-diff"
+                name: nix flake diff
+                path: post-diff
+                compression-level: 9
+                if-no-files-found: error
+                retention-period: 5
             - name: Write PR body template
               uses: https://github.com/DamianReeves/write-file-action@v1.3
               with:
@@ -63,9 +67,8 @@ jobs:
                     ${{ env.UPDATE_LOG }}
                     ```
 
-                    ```
-                    ${{ steps.read_file.outputs.contents }}
-                    ```
+                    Flake Diff can be found here:
+                    ${{ steps.upload-diff.outputs.artifact-url }}
 
                     Auto-generated by [update.yml][1] with the help of
                     [create-pull-request][2].