refactor(tvix): getEnv(): Return std::optional

This allows distinguishing between an empty value and no value.

Patch ported from upstream at
ba87b08f85

Change-Id: I061cc8e16b1a7a0341adfc3b0edca1c0c51d5c97
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1884
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>

third_party/nix/src/libexpr/primops.cc

@@ -413,7 +413,7 @@ static void prim_getEnv(EvalState& state, const Pos& pos, Value** args,
   std::string name = state.forceStringNoCtx(*args[0], pos);
   mkString(v, evalSettings.restrictEval || evalSettings.pureEval
                   ? ""
-                  : getEnv(name));
+                  : getEnv(name).value_or(""));
 }
 
 /* Evaluate the first argument, then return the second argument. */