GH Actions: run actions/setup-go@v5 before golangci-lint

This is recommended by the `golangci-lint` README, see
https://github.com/golangci/golangci-lint-action/blob/e637d2bbe4e0571eb57bfc3615ba14992312228d/README.md#compatibility:

> * `v4.0.0+` requires an explicit setup-go installation step before
>   using this action: `uses: actions/setup-go@v5`. The
>   `skip-go-installation` option has been removed.

`go mod download` is also run before `golangci-lint` just in case
because it fixed some people's problems (see
golangci/golangci-lint-action#902 (comment)).
It is also re-added to the "Test" job, because it turned out that
`actions/setup-go@v5` doesn't download dependencies as I thought it
would.

.github/workflows/go.yml

@@ -18,6 +18,8 @@ jobs:
         with:
           go-version: '1.21'
 
+      - run: go mod download
+
       - name: Test
         run: go test -v ./...
 
@@ -28,6 +30,13 @@ jobs:
 
       - uses: actions/checkout@v4
 
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.21'
+          cache: false
+
+      - run: go mod download
+
       - name: Run golangci-lint
         uses: golangci/golangci-lint-action@v4
         with: