feat: restore nix flake output in the commit description

Signed-off-by: andrea11 <[email protected]>

Author: hackardoX

action.yml

@@ -34,7 +34,7 @@ inputs:
       Automated changes by the [update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock) GitHub Action.
 
       ```
-      {{ env.GIT_COMMIT_MESSAGE }}
+      {{ env.FLAKE_UPDATE_OUTPUT }}
       ```
 
       ### Running GitHub Actions on this PR
@@ -178,16 +178,6 @@ runs:
         path: pr_body.template
         contents: ${{ inputs.pr-body }}
       env: {}
-    - name: Set additional env variables (GIT_COMMIT_MESSAGE)
-      shell: bash
-      env:
-        COMMIT_MESSAGE: ${{ inputs.commit-msg }}
-      run: |
-        DELIMITER=$(dd if=/dev/urandom bs=15 count=1 status=none | base64)
-        echo "GIT_COMMIT_MESSAGE<<$DELIMITER" >> $GITHUB_ENV
-        echo "$COMMIT_MESSAGE" >> $GITHUB_ENV
-        echo "$DELIMITER" >> $GITHUB_ENV
-        echo "GIT_COMMIT_MESSAGE is: ${COMMIT_MESSAGE}"
     - name: Interpolate PR Body
       uses: pedrolamas/handlebars-action@2995d7eadacbc8f2f6ab8431a01d84a5fa3b8bb4 # v2.4.0
       with:
@@ -211,7 +201,10 @@ runs:
         base: ${{ inputs.base }}
         branch: ${{ inputs.branch }}
         delete-branch: true
-        commit-message: ${{ env.GIT_COMMIT_MESSAGE }}
+        commit-message: |
+          ${{ inputs.commit-msg }}
+
+          ${{ env.FLAKE_UPDATE_OUTPUT }}
         committer: ${{ env.GIT_COMMITTER_NAME }} ${{ env.GIT_COMMITTER_EMAIL }}
         author: ${{ env.GIT_AUTHOR_NAME }} ${{ env.GIT_AUTHOR_EMAIL }}
         title: ${{ inputs.pr-title }}