ci(snap): build snaps natively instead of on launchpad (#38312)

The nightly snap build fails with `snapcraft remote-build`'s
`BadRequest()`: it force-pushes Gitea's full history to a fresh
Launchpad repo each run and creates the recipe before Launchpad has
indexed the `main` ref. The race is unwinnable at Gitea's repo size, and
Canonical does not support `remote-build` in CI.

Build locally on native amd64 + arm64 runners with
`snapcore/action-build` + `snapcore/action-publish` instead — no
Launchpad, no race. Drops the `LAUNCHPAD_CREDENTIALS` secret (publishing
keeps `SNAPCRAFT_STORE_CREDENTIALS`); the `snap/` recipe is unchanged,
so the same snaps ship to `latest/edge`.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

.github/workflows/release-nightly-snapcraft.yml

@@ -6,46 +6,30 @@ on:
       - main
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 permissions:
   contents: read
 
 jobs:
   build-and-publish:
-    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        runner: [ubuntu-24.04, ubuntu-24.04-arm]
+    runs-on: ${{ matrix.runner }}
 
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
         with:
           fetch-depth: 0
-
-      - name: Install snapcraft
-        run: sudo snap install snapcraft --classic
-
-      - name: Remote build
-        # remote-build runs on Launchpad and authenticates via a credentials file,
-        # not the Snap Store credentials used by upload
-        shell: bash
-        env:
-          LAUNCHPAD_CREDENTIALS: ${{ secrets.LAUNCHPAD_CREDENTIALS }}
-        run: |
-          set -euo pipefail
-          install -d -m 700 "$HOME/.local/share/snapcraft"
-          printf '%s' "$LAUNCHPAD_CREDENTIALS" > "$HOME/.local/share/snapcraft/launchpad-credentials"
-          chmod 600 "$HOME/.local/share/snapcraft/launchpad-credentials"
-          snapcraft remote-build \
-            --launchpad-accept-public-upload \
-            --build-for=amd64,arm64
-
-      - name: List built snaps
-        run: find . -maxdepth 1 -type f -name '*.snap' -print
-
-      - name: Upload and release snapcraft nightly build
+      - uses: snapcore/action-build@3bdaa03e1ba6bf59a65f84a751d943d549a54e79 # v1.3.0
+        id: build
+      - uses: snapcore/action-publish@214b86e5ca036ead1668c79afb81e550e6c54d40 # v1.2.0
         env:
           SNAPCRAFT_STORE_CREDENTIALS: ${{ secrets.SNAPCRAFT_STORE_CREDENTIALS }}
-        run: |
-          set -euo pipefail
-
-          for snap in ./*.snap; do
-            echo "Uploading $snap to edge"
-            snapcraft upload --release="latest/edge" "$snap"
-          done
+        with:
+          snap: ${{ steps.build.outputs.snap }}
+          release: latest/edge