Fix sandbox fallback settings

The tmpDirInSandbox is different when in sandboxed vs. non-sandboxed.
Since we don’t know ahead of time here whether sandboxing is enabled,
we need to reset all of the env vars we’ve set previously. This fixes
the issue encountered in https://github.com/NixOS/nixpkgs/issues/70856.

src/libstore/build.cc

@@ -956,6 +956,9 @@ private:
     /* Fill in the environment for the builder. */
     void initEnv();
 
+    /* Setup tmp dir location. */
+    void initTmpDir();
+
     /* Write a JSON file containing the derivation attributes. */
     void writeStructuredAttrs();
 
@@ -2383,6 +2386,7 @@ void DerivationGoal::startBuilder()
         if (res != 0 && settings.sandboxFallback) {
             useChroot = false;
             tmpDirInSandbox = tmpDir;
+            initTmpDir();
             goto fallback;
         } else if (res != 0)
             throw Error("unable to start build process");
@@ -2438,32 +2442,7 @@ void DerivationGoal::startBuilder()
 }
 
 
-void DerivationGoal::initEnv()
+void DerivationGoal::initTmpDir() {
-{
-    env.clear();
-
-    /* Most shells initialise PATH to some default (/bin:/usr/bin:...) when
-       PATH is not set.  We don't want this, so we fill it in with some dummy
-       value. */
-    env["PATH"] = "/path-not-set";
-
-    /* Set HOME to a non-existing path to prevent certain programs from using
-       /etc/passwd (or NIS, or whatever) to locate the home directory (for
-       example, wget looks for ~/.wgetrc).  I.e., these tools use /etc/passwd
-       if HOME is not set, but they will just assume that the settings file
-       they are looking for does not exist if HOME is set but points to some
-       non-existing path. */
-    env["HOME"] = homeDir;
-
-    /* Tell the builder where the Nix store is.  Usually they
-       shouldn't care, but this is useful for purity checking (e.g.,
-       the compiler or linker might only want to accept paths to files
-       in the store or in the build directory). */
-    env["NIX_STORE"] = worker.store.storeDir;
-
-    /* The maximum number of cores to utilize for parallel building. */
-    env["NIX_BUILD_CORES"] = (format("%d") % settings.buildCores).str();
-
     /* In non-structured mode, add all bindings specified in the
        derivation via the environment, except those listed in the
        passAsFile attribute. Those are passed as file names pointing
@@ -2501,6 +2480,35 @@ void DerivationGoal::initEnv()
        inode of the current directory doesn't appear in .. (because
        getdents returns the inode of the mount point). */
     env["PWD"] = tmpDirInSandbox;
+}
+
+void DerivationGoal::initEnv()
+{
+    env.clear();
+
+    /* Most shells initialise PATH to some default (/bin:/usr/bin:...) when
+       PATH is not set.  We don't want this, so we fill it in with some dummy
+       value. */
+    env["PATH"] = "/path-not-set";
+
+    /* Set HOME to a non-existing path to prevent certain programs from using
+       /etc/passwd (or NIS, or whatever) to locate the home directory (for
+       example, wget looks for ~/.wgetrc).  I.e., these tools use /etc/passwd
+       if HOME is not set, but they will just assume that the settings file
+       they are looking for does not exist if HOME is set but points to some
+       non-existing path. */
+    env["HOME"] = homeDir;
+
+    /* Tell the builder where the Nix store is.  Usually they
+       shouldn't care, but this is useful for purity checking (e.g.,
+       the compiler or linker might only want to accept paths to files
+       in the store or in the build directory). */
+    env["NIX_STORE"] = worker.store.storeDir;
+
+    /* The maximum number of cores to utilize for parallel building. */
+    env["NIX_BUILD_CORES"] = (format("%d") % settings.buildCores).str();
+
+    initTmpDir();
 
     /* Compatibility hack with Nix <= 0.7: if this is a fixed-output
        derivation, tell the builder, so that for instance `fetchurl'