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/libstore/ssh.cc

@@ -22,8 +22,8 @@ SSHMaster::SSHMaster(const std::string& host, std::string keyFile,
 
 void SSHMaster::addCommonSSHOpts(Strings& args) {
   for (auto& i :
-       absl::StrSplit(getEnv("NIX_SSHOPTS"), absl::ByAnyChar(" \t\n\r"),
-                      absl::SkipEmpty())) {
+       absl::StrSplit(getEnv("NIX_SSHOPTS").value_or(""),
+                      absl::ByAnyChar(" \t\n\r"), absl::SkipEmpty())) {
     args.push_back(std::string(i));
   }
   if (!keyFile.empty()) {