abuild: make default_prepare() always end up in "$builddir"

So far default_prepare() ended up in "$builddir" only if there were any
patches.  It's better to be more consistent and make such change always.

abuild.in

@@ -565,10 +565,10 @@ have_patches() {
 
 default_prepare() {
 	local i
+	cd "$builddir" || { error "Is \$builddir set correctly?"; return 1; }
 	if ! have_patches; then
 		return 0
 	fi
-	cd "$builddir" || { error "Is \$builddir set correctly?"; return 1; }
 	for i in $source; do
 		case $i in
 			*.patch)