Bug 13379: Allow using NSS to sign and verify MAR signatures

Allow using NSS on all platforms for checking MAR signatures (instead
  of using OS-native APIs, the default on Mac OS and Windows).
  So that the NSS and NSPR libraries the updater depends on can be
  found at runtime, we add the firefox directory to the shared library
  search path on macOS.
  On Linux, rpath is used to solve that problem, but that approach
  won't work on macOS because the updater executable is copied during
  the update process to a location that can vary.