Turn checkbashisms into a make target

make_gitrev.sh actually breaks checkbashisms' parser,
which /insists/ that the end-of-line " is actually a string start

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12101

scripts/make_gitrev.sh

@@ -56,9 +56,9 @@ then
 	# already exists (in the source)
 	if [ -f "${top_srcdir}/${GITREV}" ]
 	then
-		ZFS_GITREV="$(sed -n \
+		ZFS_GITREV=$(sed -n \
 			'1s/^#define[[:blank:]]ZFS_META_GITREV "\([^"]*\)"$/\1/p' \
-			"${top_srcdir}/${GITREV}")"
+			"${top_srcdir}/${GITREV}")
 	fi
 elif [ ${dist} = yes ]
 then

scripts/paxcheck.sh

@@ -1,7 +1,6 @@
 #!/bin/sh
 
-# shellcheck disable=SC2039
-if ! type scanelf > /dev/null 2>&1; then
+if ! command -v scanelf > /dev/null; then
     echo "scanelf (from pax-utils) is required for these checks." >&2
     exit 3
 fi