archlinux: fix incorrect keyring being populated

Fix one of the issue described in the following commit:
https://github.com/QubesOS/qubes-issues/issues/3185

archlinux/PKGBUILD-keyring.install

@@ -1,6 +1,6 @@
 post_upgrade() {
 	if usr/bin/pacman-key -l >/dev/null 2>&1; then
-		usr/bin/pacman-key --populate archlinux
+		usr/bin/pacman-key --populate qubesos-vm
 	fi
 	release=$(echo "$1" | cut -d '.' -f 1,2)