feat(zfs): mount dedicated Nix build dataset

Mount root_pool/nix_build at /nix/var/nix/builds on existing hosts and
generate the same layout for new installations.

python/installer/__main__.py

@@ -264,7 +264,7 @@ def create_nix_hardware_file(mnt_dir: str, disks: Sequence[str], encrypt: str |
         '    "/home" = {\n      device = "root_pool/home";\n      fsType = "zfs";\n    };\n\n'
         '    "/var" = {\n      device = "root_pool/var";\n      fsType = "zfs";\n    };\n\n'
         '    "/nix" = {\n      device = "root_pool/nix";\n      fsType = "zfs";\n    };\n\n'
-        '    "/nix_build" = {\n      device = "root_pool/nix_build";\n      fsType = "zfs";\n    };\n\n'
+        '    "/nix/var/nix/builds" = {\n      device = "root_pool/nix_build";\n      fsType = "zfs";\n    };\n\n'
         '    "/boot" = {\n'
         f'      device = "/dev/disk/by-uuid/{get_boot_drive_id(disks[0])}";\n'
         '      fsType = "vfat";\n      options = [\n        "fmask=0077"\n'
@@ -287,7 +287,7 @@ def install_nixos(mnt_dir: str, disks: Sequence[str], encrypt: str | None) -> No
     run_output(("mount", "-o", "X-mount.mkdir", "-t", "zfs", "root_pool/home", f"{mnt_dir}/home"))
     run_output(("mount", "-o", "X-mount.mkdir", "-t", "zfs", "root_pool/var", f"{mnt_dir}/var"))
     run_output(("mount", "-o", "X-mount.mkdir", "-t", "zfs", "root_pool/nix", f"{mnt_dir}/nix"))
-    run_output(("mount", "-o", "X-mount.mkdir", "-t", "zfs", "root_pool/nix_build", f"{mnt_dir}/nix_build"))
+    run_output(("mount", "-o", "X-mount.mkdir", "-t", "zfs", "root_pool/nix_build", f"{mnt_dir}/nix/var/nix/builds"))
 
     for disk in disks:
         run_output(("mkfs.vfat", "-n", "EFI", f"{disk}-part1"))

systems/bob/hardware.nix

@@ -50,6 +50,11 @@
       fsType = "zfs";
     };
 
+    "/nix/var/nix/builds" = {
+      device = "root_pool/nix_build";
+      fsType = "zfs";
+    };
+
     "/var" = {
       device = "root_pool/var";
       fsType = "zfs";

systems/brain/hardware.nix

@@ -53,6 +53,11 @@
       fsType = "zfs";
     };
 
+    "/nix/var/nix/builds" = {
+      device = "root_pool/nix_build";
+      fsType = "zfs";
+    };
+
     "/boot" = {
       device = "/dev/disk/by-uuid/12CE-A600";
       fsType = "vfat";

systems/jeeves/hardware.nix

@@ -96,6 +96,11 @@ in
       fsType = "zfs";
     };
 
+    "/nix/var/nix/builds" = {
+      device = "root_pool/nix_build";
+      fsType = "zfs";
+    };
+
     "/var" = {
       device = "root_pool/var";
       fsType = "zfs";

systems/rhapsody-in-green/hardware.nix

@@ -34,14 +34,22 @@
       device = "root_pool/root";
       fsType = "zfs";
     };
+
     "/home" = {
       device = "root_pool/home";
       fsType = "zfs";
     };
+
+    "/nix/var/nix/builds" = {
+      device = "root_pool/nix_build";
+      fsType = "zfs";
+    };
+
     "/var" = {
       device = "root_pool/var";
       fsType = "zfs";
     };
+
     "/boot" = {
       device = "/dev/disk/by-uuid/12CE-A600";
       fsType = "vfat";