xbps-src: cleaned up usage of mktemp(1).

This one was on my TODO for a long while... fixes unportable
arguments (checked with the NetBSD's manual page).

common/hooks/pre-pkg/04-generate-runtime-deps.sh

@@ -61,7 +61,7 @@ hook() {
         return 0
     fi
 
-    depsftmp=$(mktemp -t xbps_src_depstmp.XXXXXXXXXX) || return 1
+    depsftmp=$(mktemp || exit 1)
     find ${PKGDESTDIR} -type f -perm -u+w > $depsftmp 2>/dev/null
 
     exec 3<&0 # save stdin