CI: don't use go test -race on Windows

It's just too slow, at 6m when linux does more and only takes 1m.
Luckily, the vast majority of our code is not GOOS-specific,
so running the race detector on linux is most likely enough.

This should cut down Windows builds by over two minutes,
as that's how long `go test -race` takes there.
For consistency, drop -race on Mac as well.

.github/workflows/test.yml

@@ -12,9 +12,11 @@ jobs:
       with:
         go-version: ${{ matrix.go-version }}
     - uses: actions/checkout@v3
+
     - run: go test ./...
-    - run: go test -race ./...
 
+    - run: go test -race ./...
+      if: matrix.os == 'ubuntu-latest'
     - run: GOARCH=386 go test -count=1 ./...
       if: matrix.os == 'ubuntu-latest'
     - name: confirm tests with Bash 5.1