chore(3p): obtain home-manager source from upstream directly

Instead of waiting for nixpkgs to [update home-manager], we can track
the upstream repository directly (using master as a channel seems to
be common practice) and overlay the up to date source into our nixpkgs
instance.

For //users/tazjin/home and //users/wpcarro/nixos/marcus, we need to
set `home.stateVersion`, since the default value for this option was
removed some time this summer.

[update home-manager]: https://github.com/NixOS/nixpkgs/issues/197907

Change-Id: I8c153fb7d3b55a4040652a5d619761b640119105
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7098
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

third_party/overlays/tvl.nix

@@ -28,6 +28,12 @@ in
 
   nix_latest = super.nix;
 
+  home-manager = super.home-manager.overrideAttrs (_: {
+    src = depot.third_party.sources.home-manager;
+    version = "git-"
+      + builtins.substring 0 7 depot.third_party.sources.home-manager.rev;
+  });
+
   clang-tools_11 = self.clang-tools.override {
     llvmPackages = self.llvmPackages_11;
   };