CI checkstyle: pin ubuntu version

Our checkstyle doesn't work well on Ubuntu 20.04,
temporary pin it to 18.04.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: George Melikov <[email protected]>
Closes openzfs#11713

Author: gmelikov

.github/workflows/checkstyle.yaml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   checkstyle:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2
       with: