Fix CI build cache on macos.

- actions/cache#403
- rust-lang/cargo#8603

Fun stuff.

.github/workflows/validate-commit.yml

@@ -24,6 +24,13 @@ jobs:
           target/debug
         key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
     - run: cargo test
+    # There is a bug with BSD tar on macOS where the first 8MB of the file are
+    # sometimes all NUL bytes. See https://github.com/actions/cache/issues/403
+    # and https://github.com/rust-lang/cargo/issues/8603 for some more
+    # information. An alternative solution here is to install GNU tar, but
+    # flushing the disk cache seems to work, too.
+    - run: sudo purge
+      if: startsWith(matrix.os, 'macos-')
 
   # lint:
   #   runs-on: ubuntu-latest