main/postmarketos-ui-retroarch: declare install= globally

Declaring this variable inside subpackage functions is discouraged: the
linter will even warn about this being wrong.

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/9173>

main/postmarketos-ui-retroarch/APKBUILD

@@ -14,6 +14,7 @@ depends="
 	polkit-elogind
 	retroarch
 	"
+install="postmarketos-ui-retroarch-openrc.post-install postmarketos-ui-retroarch-openrc.post-upgrade"
 subpackages="$pkgname-openrc"
 source="
 	retroarch.initd
@@ -28,7 +29,6 @@ package() {
 
 openrc() {
 	install_if="$pkgname=$pkgver-r$pkgrel openrc"
-	install="$subpkgname.post-install $subpkgname.post-upgrade"
 	depends="elogind-openrc"
 
 	default_openrc