fix(3p/exwm): fix EXWM build with depot sources

Something changed in the machinery and broke the overriding. I didn't
notice this, as a I was temporarily using an unpatched EXWM.

Change-Id: I1a4e8ea63bd116d86a430e680c2b631474e9a0fe
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10047
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

third_party/exwm/default.nix

@@ -0,0 +1,14 @@
+{ depot, pkgs, lib, ... }:
+
+# special dance for overriding this into the fixed-point of emacs
+# packages, but having it be separately buildable.
+
+pkgs.emacsPackages.callPackage
+  ({ trivialBuild, xelb }: trivialBuild {
+    pname = "depot-exwm";
+    version = "canon";
+    src = ./.;
+
+    packageRequires = [ xelb ];
+  })
+{ }