Make possible to override the branch name via a env variable

This is useful for the CI builds because they have a detached HEAD or
for emulating a feature that is availible only on a particular branch
(e.g pleroma_ctl downloads CI artifacts only for develop and master)

.gitlab-ci.yml

@@ -173,6 +173,7 @@ amd64:
   script: &release
     - mix deps.get --only prod
     - mkdir release
+    - PLEROMA_BUILD_BRANCH=$CI_COMMIT_REF_NAME
     - mix release --path release