CI: use sccache to speed up CI (#408)

* CI: use sccache to speed up CI

* Write changelog

* Only use sccache for non-cross targets

.github/workflows/ci.yaml

@@ -50,6 +50,11 @@ jobs:
     name: Test on each target
     needs: get-msrv
     env:
+      # use sccache
+      # It's too much of a hassle to set up sccache in cross.
+      # See https://github.com/cross-rs/cross/wiki/Recipes#sccache.
+      SCCACHE_GHA_ENABLED: ${{ matrix.cargo == 'cargo' && 'true' || 'false'}}
+      RUSTC_WRAPPER: ${{ matrix.cargo == 'cargo' && 'sccache' || '' }}
       # Emit backtraces on panics.
       RUST_BACKTRACE: 1
     runs-on: ${{ matrix.os }}
@@ -114,6 +119,12 @@ jobs:
           toolchain: ${{ matrix.rust }}
           targets: ${{ matrix.target }}
           components: clippy
+      
+      - name: Set up sccache
+        # It's too much of a hassle to set up sccache in cross.
+        # See https://github.com/cross-rs/cross/wiki/Recipes#sccache.
+        if: matrix.cargo == 'cargo'
+        uses: mozilla-actions/sccache-action@v0.0.4
 
       - name: Install cross
         if: matrix.cargo == 'cross'