mkosi: Switch to Arch Linux packaging sources main branch

https://gitlab.archlinux.org/archlinux/packaging/packages/systemd/-/merge_requests/8
was merged so let's switch branches to the main branch.

.gitmodules

@@ -16,5 +16,5 @@
 	branch = c9s-sig-hyperscale
 [submodule "pkg/arch"]
 	path = pkg/arch
-	url = https://gitlab.archlinux.org/daandemeyer/systemd.git
-	branch = strip
+	url = https://gitlab.archlinux.org/archlinux/packaging/packages/systemd
+	branch = main

mkosi.images/base/mkosi.conf.d/10-arch/mkosi.build.chroot

@@ -35,6 +35,10 @@ else
     TS="${SOURCE_DATE_EPOCH:-$(date +%s)}"
 fi
 
+sed --in-place "pkg/$PKG_SUBDIR/PKGBUILD" \
+    --expression "s/^_tag=.*/_tag=$(cat meson.version)/" \
+    --expression "s/^pkgrel=.*/pkgrel=$(date "+%Y%m%d%H%M%S" --date "@$TS")/"
+
 # We get around makepkg's root check by setting EUID to something else.
 # shellcheck disable=SC2046
 env --chdir="pkg/$PKG_SUBDIR" \
@@ -48,6 +52,4 @@ env --chdir="pkg/$PKG_SUBDIR" \
     BUILDDIR="$PWD/pkg/$PKG_SUBDIR" \
     PKGDEST="$PACKAGEDIR" \
     PKGEXT=".pkg.tar" \
-    PKGVER="$(cat meson.version)" \
-    PKGREL="$(date "+%Y%m%d%H%M%S" --date "@$TS")" \
     MESON_EXTRA_CONFIGURE_OPTIONS="-D mode=developer -D b_sanitize=${SANITIZERS:-none}"