Make sure wrong plugin manifest file is not collected from cache

Building two versions of a gerrit plugin using the same buck would make
buck use the cached manifest file from the previous build.
With this change the result of git_describe() is used to determine
archetype version, so the manifest file will be rebuilt.

Change-Id: If5e7be8d7f11b965e973172f77fe7bfc93019f3a
2 files changed