ci: Check formatting

Also moves rustup installation to a seperate workflow and
enables caching.
The sccache action required a github.com api token, so we set
all that up too.

.forgejo/actions/sccache/action.yml

@@ -0,0 +1,29 @@
+name: sccache
+description: |
+  Install sccache for caching builds in GitHub Actions.
+
+inputs:
+  token:
+    description: 'A Github PAT'
+    required: false
+
+runs:
+  using: composite
+  steps:
+  - name: Install sccache
+    uses: https://github.com/mozilla-actions/sccache-action@v0.0.9
+    with:
+      token: ${{ inputs.token }}
+  - name: Configure sccache
+    uses: https://github.com/actions/github-script@v7
+    with:
+      script: |
+        core.exportVariable('ACTIONS_RESULTS_URL', process.env.ACTIONS_RESULTS_URL || '');
+        core.exportVariable('ACTIONS_RUNTIME_TOKEN', process.env.ACTIONS_RUNTIME_TOKEN || '');
+  - shell: bash
+    run: |
+      echo "SCCACHE_GHA_ENABLED=true" >> $GITHUB_ENV
+      echo "RUSTC_WRAPPER=sccache" >> $GITHUB_ENV
+      echo "CMAKE_C_COMPILER_LAUNCHER=sccache" >> $GITHUB_ENV
+      echo "CMAKE_CXX_COMPILER_LAUNCHER=sccache" >> $GITHUB_ENV
+      echo "CMAKE_CUDA_COMPILER_LAUNCHER=sccache" >> $GITHUB_ENV