ci: build inside bread-ecosystem's shared Arch container

Ports breadpaper onto bread-ecosystem's shared, pinned Arch build
image/script instead of installing toolchain/libraries directly on the
bare self-hosted runner. Follows the pattern proven in breadpad:
ci/build.sh clones bread-ecosystem pinned to the sha in
ci/bread-ecosystem.rev and delegates to its ci/build.sh.

breadpaper needs nothing beyond the shared image's base package set
(only clap/anyhow as deps, no image-processing or GTK crates), so no
ci/deps.txt is added.

Also adds check.yml: clippy + test on feature/**/fix/** pushes, as a
fast pre-release-build signal, matching breadpaper's own build/test
flags (--release --locked, no --workspace since this isn't a
workspace).

.forgejo/workflows/rc-release.yml

@@ -21,10 +21,10 @@ jobs:
             "https://git.breadway.dev/${GITHUB_REPOSITORY}.git" src
 
       - name: build
-        run: cd src && cargo build --release --locked
+        run: cd src && bash ci/build.sh cargo build --release --locked
 
       - name: test
-        run: cd src && cargo test --release --locked
+        run: cd src && bash ci/build.sh cargo test --release --locked
 
       - name: prepare artifacts
         run: |