fix(3p/overlays): Avoid builds of mkShell derivations in CI

Change-Id: I94a728e0041c7be30675f4344b5f8e49099d4cd3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4333
Tested-by: BuildkiteCI
Autosubmit: tazjin <mail@tazj.in>
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/overlays/tvl.nix

@@ -59,4 +59,9 @@ self: super: {
   # nix-serve does not work with nix 2.4
   # https://github.com/edolstra/nix-serve/issues/28
   nix-serve = super.nix-serve.override { nix = super.nix_2_3; };
+
+  # Avoid builds of mkShell derivations in CI.
+  mkShell = super.lib.makeOverridable(args: (super.mkShell args) // {
+    meta.ci = false;
+  });
 }