Fix watchdog_test lint and test failures on macos. (prometheus#3003)

Ensure identical build flags embedded in both files.

Signed-off-by: Chris Cleeland <chris.cleeland@gmail.com>
Signed-off-by: Vitaly Zhuravlev <v-zhuravlev@users.noreply.github.com>

collector/watchdog_test.go

@@ -11,8 +11,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build !nowatchdog
-// +build !nowatchdog
+//go:build linux && !nowatchdog
+// +build linux,!nowatchdog
 
 package collector