libfaketime exceptions are not passed down the Mozilla build system properly

It seems that not all the commands passed to FAKETIME_SKIP_CMDS are passed down the Mozilla build system which leads to legacy/trac#13877 (moved) not being fixable. Quoting from the OS X Firefox descriptor:

  # XXX Order matters as for some reason the env variable has only the first
  # command assigned when it comes to the rsync parts in the build process.