fix(grfn/home): Set home.homeDirectory option

This avoids leaking $HOME from the environment into the build, which was
previously causing yerenHome to get rebuilt on every single depot CI
invocation.

Change-Id: Icb3c32e91186468e548c7b86b1a12bf9adef4fe9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5118
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/system/home/modules/common.nix

@@ -13,6 +13,8 @@
     ../modules/lib/cloneRepo.nix
   ];
 
+  home.homeDirectory = "/home/grfn";
+
   programs.password-store.enable = true;
 
   grfn.impure.clonedRepos.passwordStore = {