Revert "libstore: simplify fallback build directory logic"

Revert submission 3850

Reason for revert: caused multiple regressions noticed in https://git.lix.systems/lix-project/lix/issues/975 and https://git.lix.systems/lix-project/lix/issues/966 (suspected).

Root cause analysis has not been done yet and this breaks Lix on Darwin on HEAD.

Reverted changes: /q/submissionid:3850

Change-Id: Id6db066f3f5c454b258157bc12e0f26020de94a7

lix/libstore/build/local-derivation-goal.cc

@@ -457,6 +457,8 @@ try {
             throw;
         }
 
+        auto globalTmp = defaultTempDir();
+        createDirs(globalTmp);
 #if __APPLE__
         /* macOS filesystem namespacing does not exist, to avoid breaking builds, we need to weaken
          * the mode bits on the top-level directory. This avoids issues like
@@ -466,7 +468,7 @@ try {
         constexpr int toplevelDirMode = 0700;
 #endif
         auto nixBuildsTmp = createTempDir(
-            "", fmt("nix-builds-%s", geteuid()), false, false, toplevelDirMode
+            globalTmp, fmt("nix-builds-%s", geteuid()), false, false, toplevelDirMode
         );
         warn(
             "Failed to use the system-wide build directory '%s', falling back to a temporary "
@@ -474,10 +476,10 @@ try {
             settings.buildDir.get(),
             nixBuildsTmp
         );
+        worker.buildDirOverride = nixBuildsTmp;
         tmpDirRoot = createTempDir(
             nixBuildsTmp, "nix-build-" + std::string(drvPath.name()), false, false, 0700
         );
-        worker.buildDirOverride = nixBuildsTmp;
     }
     /* The TOCTOU between the previous mkdir call and this open call is unavoidable due to
      * POSIX semantics.*/