Publish mainline snaps to stable channel

Updating the workflow to ensure that snaps from the mainline
channel are published to the stable release. This aligns with
the release strategy for more consistent deployment practices.

.github/workflows/publish-snaps.yaml

@@ -58,11 +58,12 @@ jobs:
           git diff snap/snapcraft.yaml
 
       - uses: snapcore/action-build@v1
-        id: build
+        if: ${{ steps.metadata.outputs.release_channel == 'mainline' }}
 
       - uses: snapcore/action-publish@v1
+        if: ${{ steps.metadata.outputs.release_channel == 'mainline' }}
         env:
           SNAPCRAFT_STORE_CREDENTIALS: ${{ secrets.SNAPCRAFT_TOKEN }}
         with:
           snap: ${{ steps.build.outputs.snap }}
-          release: ${{ steps.metadata.outputs.release_channel }}
+          release: stable