fix(tvix/eval): type check function argument with set pattern

C++ Nix forces and typechecks the passed argument even if it is not
necessary in order to compute the return value of the function. I
discovered this when I thought our formals miscompilation might be that
we are too strict, but doesn't look like it in this case.

Change-Id: Ifb3c92592293052c489d1e3ae8c7c54e4b6b4dc6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8701
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/tvix_tests/eval-fail-function-formals-typecheck.nix

@@ -0,0 +1,2 @@
+# A function with formal set arguments forces its argument set and verifies its type.
+({ ... }@args: 42) [ ]