systemd: fix source= formatting

Is the common used way in Alpine, we don't have any style changes documented
in Alpine right now, but it is WIP and it would certainly documented be like that.
This then would also fix the nightly-builder replacing of source=.

[ci:skip-vercheck]: just a style change

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7818>

extra-repos/systemd/systemd/APKBUILD

@@ -154,8 +154,7 @@ subpackages="
 	$pkgname-zsh-completion
 	"
 
-source="
-	$pkgname-$pkgver.tar.gz::https://github.com/systemd/systemd/archive/refs/tags/v$_pkgver.tar.gz
+source="$pkgname-$pkgver.tar.gz::https://github.com/systemd/systemd/archive/refs/tags/v$_pkgver.tar.gz
 	wired.network
 	"
 builddir="$srcdir/systemd-$_pkgver"