Fix shellcheck issue SC2154

SC2154: var is referenced but not assigned.

The 'rootmnt' variable is set in /usr/share/initramfs-tools/init and if
it's not available, that would be bad.
According to SC2154 you can fix that issue by using '${var:?}' so it
would fail if unset or empty.
So apply that and reference that SC item in the comments.

Also remove it from the exclude list in the CI config file.

rootfs/etc/initramfs-tools/scripts/local-bottom/rpi-resizerootfs

@@ -33,8 +33,8 @@ fi
 
 log_begin_msg "$0 resizing $ROOT"
 
-# Unmount for safety
-umount "${rootmnt}"
+# Unmount for safety; fail if unset or empty (shellcheck SC2154)
+umount "${rootmnt:?}"
 
 # Expand the partition size to fill the entire device
 parted -s $rootdev resizepart $rootpart_nr 100%