archbuild: Fix a variable name mistake. ($copydir -> $copy)

It tried to lock `$copydir.lock`, which was the ONLY mention of $copydir in
the entire file.  Surely it meant `$copy.lock`; the line was probably
originally copy/pasted from makechrootpkg or similar, where $copydir is
used.

archbuild.in

@@ -49,7 +49,7 @@ if ${clean_first} || [[ ! -d "${chroots}/${repo}-${arch}" ]]; then
 		[[ -d $copy ]] || continue
 		msg2 "Deleting chroot copy '$(basename "${copy}")'..."
 
-		lock 9 "$copydir.lock" "Locking chroot copy '$copy'"
+		lock 9 "$copy.lock" "Locking chroot copy '$copy'"
 
 		if [[ "$(stat -f -c %T "${copy}")" == btrfs ]]; then
 			{ type -P btrfs && btrfs subvolume delete "${copy}"; } &>/dev/null