Merge pull request #2701 from freenas/fix-78231

[install.sh] Fix upgrades from installer

src/freenas-installer/etc/install.sh

@@ -629,7 +629,7 @@ disk_is_freenas()
 	# This code is very clumsy.  There
 	# should be a way to structure it such that
 	# all of the cleanup happens as we want it to.
-	zdb -l ${os_part} | fgrep -q "name: 'freenas-boot'" || return 1
+	zdb -l ${os_part} | grep -qF "name: 'freenas-boot'" || return 1
 	zpool import -N -f freenas-boot || return 1
 
 	# Now we want to figure out which dataset to use.