Fix making the build directory kept by keep-failed readable

Caused by 1d3696f

Without this fix the kept build directory is readable only by root

```
$ sudo ls -ld /comp-temp/nix-build-openssh-static-x86_64-unknown-linux-musl-9.8p1.drv-5
drwx------ root root 60 B Wed Sep 11 00:09:48 2024  /comp-temp/nix-build-openssh-static-x86_64-unknown-linux-musl-9.8p1.drv-5/

$ sudo ls -ld /comp-temp/nix-build-openssh-static-x86_64-unknown-linux-musl-9.8p1.drv-5/build
drwxr-xr-x nixbld1 nixbld 80 B Wed Sep 11 00:09:58 2024  /comp-temp/nix-build-openssh-static-x86_64-unknown-linux-musl-9.8p1.drv-5/build/
```

(cherry picked from commit ebebe62)

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

@@ -2961,6 +2961,7 @@ void LocalDerivationGoal::deleteTmpDir(bool force)
            might have privileged stuff (like a copy of netrc). */
         if (settings.keepFailed && !force && !drv->isBuiltin()) {
             printError("note: keeping build directory '%s'", tmpDir);
+            chmod(topTmpDir.c_str(), 0755);
             chmod(tmpDir.c_str(), 0755);
         }
         else