nixos/systemd/initrd: Set /etc/hostname in the initrd.

With the the Systemd-based initrd, systemd-journald is doing the logging.
One of Journald's Trusted Journal Fields is `_HOSTNAME` (systemd.journal-fields(7)).
Without explicitly setting the hostname via this file or the kernel cmdline, `localhost` is used and captured in the journal.
As a result, a boot's log references multiple hostnames.
With centralized log collection this breaks filtering (more so when logs from multiple Systemd-based initrds are streaming in simultaneously.

Fixes #318907.

nixos/modules/system/boot/systemd/initrd.nix

@@ -160,7 +160,7 @@ in {
       description = "Set of files that have to be linked into the initrd";
       example = literalExpression ''
         {
-          "/etc/hostname".text = "mymachine";
+          "/etc/machine-id".source = /etc/machine-id;
         }
       '';
       default = {};
@@ -444,6 +444,9 @@ in {
         "/etc/os-release".source = config.boot.initrd.osRelease;
         "/etc/initrd-release".source = config.boot.initrd.osRelease;
 
+        # For systemd-journald's _HOSTNAME field; needs to be set early, cannot be backfilled.
+        "/etc/hostname".text = config.networking.hostName;
+
       } // optionalAttrs (config.environment.etc ? "modprobe.d/nixos.conf") {
         "/etc/modprobe.d/nixos.conf".source = config.environment.etc."modprobe.d/nixos.conf".source;
       };