CI: replace from-source libadwaita build with pinned Arch container

The dev/rc/release workflows rebuilt libadwaita from source inside an
uncached Fedora container on every single run, because Ubuntu 24.04's
packaged libadwaita was too old for gtk4 0.11's v4_12 feature. That
from-source build broke repeatedly on drift (rust dep versions,
libadwaita ABI, and finally a removed meson option), producing eight
straight failed CI runs.

Arch's repos already carry current gtk4/libadwaita/gtk4-layer-shell as
prebuilt packages, and breadpad only targets BOS/Arch, so there's no
reason to build anything from source. Swap the container base to a
digest-pinned archlinux image (ci/Containerfile) with those packages
installed via pacman, and extract the shared "build the image, run
cargo inside it" logic into ci/build.sh so it isn't duplicated three
times across the workflows. Cargo's registry/git/target caches persist
in named docker volumes across runs.

Also:
- Add check.yml: clippy + test on feature/**/fix/** pushes, so lint/
  build breakage surfaces before it reaches main and triggers a
  dev-track release.
- Fix release.yml's tag trigger (tags-ignore) so an rc tag push no
  longer also spawns a skipped release.yml run alongside rc-release.yml.

Verified locally: the container builds cleanly via pacman (no source
compilation), and `cargo build --release --locked --workspace` inside
it produces working breadpad/breadman binaries linked against
libgtk4-layer-shell.so.0 and libadwaita-1.so.0 respectively.

.forgejo/workflows/check.yml

@@ -0,0 +1,24 @@
+name: check
+
+# Fast-fail lint/test on short-lived work branches, before it ever reaches
+# main and triggers a dev-track release build.
+on:
+  push:
+    branches: ['feature/**', 'fix/**']
+
+jobs:
+  check:
+    runs-on: [self-hosted, hestia]
+    steps:
+      - name: checkout
+        run: |
+          set -euo pipefail
+          rm -rf src && mkdir src
+          git clone --branch "${GITHUB_REF_NAME}" --depth 1 \
+            "https://git.breadway.dev/${GITHUB_REPOSITORY}.git" src
+
+      - name: clippy
+        run: cd src && bash ci/build.sh cargo clippy --workspace --all-targets --locked -- -D warnings
+
+      - name: test
+        run: cd src && bash ci/build.sh cargo test --workspace --locked