feat(tvix/eval): Implement inherit from outer scope in attrs

Change-Id: I97fa45059b7f2cbe96eb60bd1821e3e25832df66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6212
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-attrs-simple-inherit.exp

@@ -0,0 +1 @@
+{ a = 1; }

tvix/eval/src/tests/tvix_tests/eval-okay-attrs-simple-inherit.nix

@@ -0,0 +1,4 @@
+let
+  a = 1;
+in
+{ inherit a; }