ci: properly cache everything

Author: pietroalbini

.github/workflows/main.yml

@@ -11,13 +11,20 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Cache cargo
+
+      - name: Detect the installed Rust version
+        id: rustc
+        run: echo "::set-output name=version::$(rustc -V)"
+
+      - name: Cache Cargo's target directory
         uses: actions/cache@v2
         with:
           path: target
-          key: cargo-build-${{ hashFiles('**/Cargo.lock') }}
+          key: cargo-build-${{ steps.rustc.outputs.version }}-${{ hashFiles('**/Cargo.lock') }}-${{ hashFiles('src/**') }}
           restore-keys: |
-            cargo-build-
+            cargo-build-${{ steps.rustc.outputs.version }}-${{ hashFiles('**/Cargo.lock') }}-
+            cargo-build-${{ steps.rustc.outputs.version }}-
+
       - run: cargo run
       - run: cp CNAME ./site/
       - run: touch site/.nojekyll