Update checkstyle workflow env to ubuntu-20.04
- checkstyle workflow uses ubuntu-20.04 environment
- improved mancheck.sh readability
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12713