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

scripts/mancheck.sh

@@ -11,7 +11,9 @@
 # AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
 # OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 #
-# shellcheck disable=SC2086
+# shellcheck disable=SC2086,SC2250
+
+trap 'rm -f "$stdout_file" "$stderr_file" "$result_file"' EXIT
 
 if [ "$#" -eq 0 ]; then
     echo "Usage: $0 manpage-directory..."
@@ -25,7 +27,6 @@ fi
 
 IFS="
 "
-
 files="$(find "$@" -type f -name '*[1-9]*')" || exit 1
 
 add_excl="$(awk '
@@ -38,6 +39,15 @@ add_excl="$(awk '
 
 # Redirect to file instead of 2>&1ing because mandoc flushes inconsistently(?) which tears lines
 # https://github.com/openzfs/zfs/pull/12129/checks?check_run_id=2701608671#step:5:3
-etmp="$(mktemp)"
-! { mandoc -Tlint $files 2>"$etmp"; cat "$etmp"; rm -f "$etmp"; } |
-    grep -vE -e 'mandoc: outdated mandoc.db' -e 'STYLE: referenced manual not found' $add_excl >&2
+stdout_file="$(mktemp)"
+stderr_file="$(mktemp)"
+mandoc -Tlint $files 1>"$stdout_file" 2>"$stderr_file"
+result_file="$(mktemp)"
+grep -vhE -e 'mandoc: outdated mandoc.db' -e 'STYLE: referenced manual not found' $add_excl "$stdout_file" "$stderr_file" > "$result_file"
+
+if [ -s "$result_file" ]; then
+    cat "$result_file"
+    exit 1
+else
+    echo "no errors found"
+fi