ci: Add shared cache of registry info and sources

Should hopefully speed up compiling of the old versions.

.github/workflows/test.yml

@@ -99,6 +99,15 @@ jobs:
         with:
           toolchain: ${{ matrix.rust }}
           components: clippy
+
+      - uses: actions/cache@v3
+        name: Cache ~/.cargo registry info and sources
+        with:
+          key: cratesio-always-shared
+          path: |
+            ~/.cache/registry
+            ~/.cache/git
+
       - name: Check
         # A failing `cargo check` always ends the build
         run: |