Reapply "Now that NixOS/nixpkgs#282022 has landed, switch to gpt-auto…

… for the forseeable."

This reverts commit d3bb46c.
Per Matrix discussion, we're going to add in `rw` to the kernel arguments.

boot.nix

@@ -27,7 +27,6 @@ in
 
   boot.initrd.availableKernelModules = [
     "ahci"
-    "efivarfs"
     "nvme"
     "sd_mod"
     "usb_storage"
@@ -49,17 +48,13 @@ in
   # No software RAID in this system.
   boot.swraid.enable = false;
 
-  fileSystems = {
-    "/" = {
-      device = "/dev/disk/by-uuid/d5a0f038-5839-44e1-ad49-5edd00d7f81e";
-      fsType = "ext4";
-    };
+  # Use `systemd-gpt-auto-root` to detect the root filesystem partition.
+  boot.initrd.supportedFilesystems = [ "ext4" ];
+  boot.initrd.systemd.root = "gpt-auto";
 
-    "/boot" = {
-      device = "/dev/disk/by-uuid/0D2C-FF36";
-      fsType = "vfat";
-    };
-  };
+  # Mount the boot partition specifically. I'd like to move this to a mount unit.
+  fileSystems."/boot".device = "/dev/disk/by-uuid/0D2C-FF36";
+  fileSystems."/boot".fsType = "vfat";
 
   # No swap devices in this system (maybe a bad call.)
   swapDevices = [ ];