ci: prevent concurrency on docker/rust actions (#461)

.github/workflows/rust.yaml

@@ -5,6 +5,10 @@ on:
     branches: [main]
   pull_request:
 
+concurrency: 
+  group: ${{ github.head_ref }}
+  cancel-in-progress: true
+
 env:
   CARGO_TERM_COLOR: always
 
@@ -14,6 +18,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - name: Free up disk space
+        run: |
+          sudo rm -rf /usr/local/lib/android /usr/share/dotnet /opt/ghc
+
       - name: Install latest stable
         uses: actions-rs/toolchain@v1
         with: