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/nix-prefetch-url/nix-prefetch-url.cc

@@ -59,7 +59,7 @@ static int _main(int argc, char** argv) {
   {
     HashType ht = htSHA256;
     std::vector<std::string> args;
-    bool printPath = !getEnv("PRINT_PATH").empty();
+    bool printPath = getEnv("PRINT_PATH").has_value();
     bool fromExpr = false;
     std::string attrPath;
     bool unpack = false;