abuild: set default builddir when not defined in APKBUILD

abuild.in

@@ -568,8 +568,7 @@ default_prepare() {
 	if ! have_patches; then
 		return 0
 	fi
-	cd "${builddir:-$srcdir/$pkgname-$pkgver}" \
-		|| { error "Is \$builddir set correctly?"; return 1; }
+	cd "$builddir" || { error "Is \$builddir set correctly?"; return 1; }
 	for i in $source; do
 		case $i in
 			*.patch)
@@ -2239,6 +2238,8 @@ export CC
 cd "$startdir" || die
 . "$APKBUILD"
 
+builddir=${builddir:-"$srcdir/$pkgname-$pkgver"}
+
 # If REPODEST is set then it will override the PKGDEST
 if [ -z "$REPODEST" ]; then
 	warning "REPODEST is not set and is now required. Defaulting to ~/packages"