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.

.gitlab-ci.yml

@@ -35,6 +35,7 @@ docs-build:
   - develop@pleroma/pleroma
   variables:
     MIX_ENV: dev
+    PLEROMA_BUILD_ENV: prod
   script:
     - mix deps.get
     - mix compile