Only benchmark on the latest Go version, take 2

I thought #88 would do this, but apparently it didn't.

Attempting to use `go.mod` location since that means we need to update one less place on Go version release.

Skipping PR process since
(0) I don't know how to test this without landing
(1) this affects only builds
(2) the `main` looks like it's broken, which is sad

.github/workflows/benchmark.yaml

@@ -13,6 +13,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-go@v3
         with:
+          go-version-file: 'go.mod'
           check-latest: true
           cache: true
       - name: Benchmark