nixos/etc: wantedBy -> requiredBy for mounting overlay etc

I've observed that sometimes the overlay mount unit does not get started
when using wantedBy. requiredBy makes this relationship stricter and if
necessary will restart the initrd-fs.target and thus ensure that when
this target is reached /etc has alredy been mounted. This is in line
with the description of initrd-fs.target in systemd.special:

> Thus, once this target is reached the /sysroot/ hierarchy is fully set up

nixos/modules/system/etc/etc-activation.nix

@@ -64,7 +64,7 @@
             ] ++ lib.optionals (!config.system.etc.overlay.mutable) [
               "ro"
             ]);
-            wantedBy = [ "initrd-fs.target" ];
+            requiredBy = [ "initrd-fs.target" ];
             before = [ "initrd-fs.target" ];
             requires = lib.mkIf config.system.etc.overlay.mutable [ "rw-etc.service" ];
             after = lib.mkIf config.system.etc.overlay.mutable [ "rw-etc.service" ];