.github: fix staticcheck linting (#599)

Update the test.yml lint job to run linting with Go 1.25, and use the
ambient Go command to run staticcheck. This seems to fix a mysterious
bug where staticcheck was failing, presumably in the Go installation
step. Using the ambient Go command resulted in staticcheck failures,
since some symbols are detected as unused at Go 1.23 due to build tags.
Upgrading the Go version for this job avoids those errors.

Also, use a pinned version of staticcheck (as recommended), to avoid
future failures when new versions are released.

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 # v5
         with:
-          go-version: "^1.23"
+          go-version: "^1.25"
       - name: Check formatting
         run: |
           unformatted=$(gofmt -l .)
@@ -33,7 +33,8 @@ jobs:
       - name: Run staticcheck
         uses: dominikh/staticcheck-action@024238d2898c874f26d723e7d0ff4308c35589a2 # v1
         with:
-          version: "latest"
+          install-go: false
+          version: "v0.6.1"
 
   test:
     runs-on: ubuntu-latest