ci: major overhaul

- Simplify some workflow steps and auxiliary scripts
- Build library using cmake when not cross-compiling
- Try to fetch fixtures from cache first
- Use `actions-rust-lang/setup-rust-toolchain`

.github/workflows/backport.yml

@@ -1,26 +1,29 @@
-name: backport
+name: Backport Pull Request
+
 on:
   pull_request_target:
     types: [closed, labeled]
+
+permissions:
+  contents: write
+  pull-requests: write
+
 jobs:
   backport:
-    permissions:
-      contents: write
-      pull-requests: write
-    name: Backport Pull Request
     if: github.event.pull_request.merged
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - name: Checkout repository
+        uses: actions/checkout@v4
 
-      - uses: actions/create-github-app-token@v1
+      - name: Create app token
+        uses: actions/create-github-app-token@v1
         id: app-token
         with:
           app-id: ${{ vars.BACKPORT_APP }}
           private-key: ${{ secrets.BACKPORT_KEY }}
 
       - name: Create backport PR
-        id: backport
         uses: korthout/backport-action@v3
         with:
           pull_title: "${pull_title}"