scripts/check_license_headers.sh: delete, rewrite as a Go test

Updates tailscale/corp#29650

Change-Id: Iad4e4ccd9d68ebb1d1a12f335cc5295d0bd05b60
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/test.yml

@@ -702,11 +702,23 @@ jobs:
 
   licenses:
     runs-on: ubuntu-24.04
+    needs: gomod-cache
     steps:
     - name: checkout
       uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      with:
+        path: src
+    - name: Restore Go module cache
+      uses: actions/cache/restore@5a3ec84eff668545956fd18022155c47e93e2684 # v4.2.3
+      with:
+        path: gomodcache
+        key: ${{ needs.gomod-cache.outputs.cache-key }}
+        enableCrossOsArchive: true
     - name: check licenses
-      run: ./scripts/check_license_headers.sh .
+      working-directory: src
+      run: |
+        grep -q TestLicenseHeaders *.go || (echo "Expected a test named TestLicenseHeaders"; exit 1)
+        ./tool/go test -v -run=TestLicenseHeaders
 
   staticcheck:
     runs-on: ubuntu-24.04