Fix building many things in nix.

Nix 2.30.0 (2025-07-07) changed the build directory from /tmp to /nix/var/nix/builds which broke a number of builds because my ZFS datasets were utf8only. This adds a role to mount tmpfs to /nix/var/nix/builds to restore the old behavior.

nix/configuration/configuration.nix

@@ -14,6 +14,7 @@
     ./roles/base
     ./roles/bluetooth
     ./roles/boot
+    ./roles/build_in_ram
     ./roles/chromecast
     ./roles/chromium
     ./roles/d2
@@ -177,7 +178,6 @@
         (final: prev: {
           grub2 = (final.callPackage ./package/grub { });
         })
-        (disableTests "git")
       ];
 
     # This option defines the first version of NixOS you have installed on this particular machine,