testing/pipexec: fix build on riscv64

testing/pipexec/APKBUILD

@@ -12,6 +12,11 @@ source="https://github.com/flonatel/pipexec/releases/download/$pkgver/$pkgname-$
 	dont-require-bash.patch"
 builddir="$srcdir/$pkgname-$pkgver"
 
+prepare() {
+	default_prepare
+	update_config_sub
+}
+
 build() {
 	cd "$builddir"