calamares-nixos-extension: Set build-dir during nixos-install

Author: ElvishJerricco

pkgs/by-name/ca/calamares-nixos-extensions/src/modules/nixos/main.py

@@ -797,7 +797,16 @@ def run():
             "nixos-install",
             "--no-root-passwd",
             "--root",
-            root_mount_point
+            root_mount_point,
+            # Nix requires its build directory to have no
+            # world-writable parent directories. The chroot store that
+            # nixos-install uses will use the state dir in the chroot
+            # for the build-dir, but the chroot is under /tmp, which
+            # is writable. It doesn't have to be in the chroot though,
+            # so we can just realign it with the host state dir.
+            "--option",
+            "build-dir",
+            "/nix/var/nix/builds",
         ]
     )