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

mix.exs

@@ -207,6 +207,15 @@ defmodule Pleroma.Mixfile do
 
     env_name = if Mix.env() != :prod, do: to_string(Mix.env())
 
+    env_override = System.get_env("PLEROMA_BUILD_ENV")
+
+    env_name =
+      if env_override do
+        if env_override != "prod", do: env_override
+      else
+        env_name
+      end
+
     build =
       [build_name, env_name]
       |> Enum.filter(fn string -> string && string != "" end)