fix(tvix/config): properly handle nonexistent config files

If the global nix config, or any available user nix config location, does not exist, then the loadConfFile() function will throw and not finish initalizing the Nix configuration.

Change-Id: I6db83bca54d9b66699356d107720603476e32c23
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1657
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libstore/globals.cc

@@ -1,6 +1,7 @@
 #include "libstore/globals.hh"
 
 #include <algorithm>
+#include <filesystem>
 #include <map>
 #include <thread>
 
@@ -70,7 +71,9 @@ Settings::Settings()
 }
 
 void loadConfFile() {
+  if (std::filesystem::exists(settings.nixConfDir + "/nix.conf")) {
     globalConfig.applyConfigFile(settings.nixConfDir + "/nix.conf");
+  }
 
   /* We only want to send overrides to the daemon, i.e. stuff from
      ~/.nix/nix.conf or the command line. */
@@ -80,8 +83,10 @@ void loadConfFile() {
   // Iterate over them in reverse so that the ones appearing first in the path
   // take priority
   for (auto dir = dirs.rbegin(); dir != dirs.rend(); dir++) {
+    if (std::filesystem::exists(*dir + "/nix.conf")) {
       globalConfig.applyConfigFile(*dir + "/nix/nix.conf");
     }
+  }
 }
 
 unsigned int Settings::getDefaultCores() {