Bug 40075: Use /tmp for nightly builds on tb-build-06

rbm-config/tb-build-06.torproject.org.rbm.local.conf

@@ -10,7 +10,7 @@
 ### builds are made from this directory, so using a directory on a fast
 ### disk can improve build time. By default we are using a tmp directory
 ### under the tor-browser-build directory.
-#tmp_dir: /tmp
+tmp_dir: /tmp
 
 ### The debug option defines whether a debugging shell should be opened
 ### automatically in the build directory/container in case of build