Add PLEROMA_BUILD_ENV to override env for the version generator and use it in the...

Add PLEROMA_BUILD_ENV to override env for the version generator and use it in the CI for building docs

This is needed because if we run with MIX_ENV=dev we will get a "+dev"
suffix to the version in the docs, but if we run with MIX_ENV=prod the
docs build will fail because ex_doc is dev-only dependency.