Add back dirty check to the build script.

[kwight]

git describe --always --long --dirty was originally used as a version for iframed notifications on WordPress.com, but the --dirty was recently dropped from the build script since the result was always dirty (building on Linux will cause a fsevents dependency to be skipped):

npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.1.3 (node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.1.3: wanted {"os":"darwin","arch":"any"} (current: {"os":"linux","arch":"x64"})

However, it's great to know if a deploy was based on a dirty build, since it can indicate changes were made from the officially-tagged release. Let's see if we can get the result of the dirty flag to be meaningful again.