fix(ci): Populate elixir vsn from env at build time (#7773)

Dependabot's workflow is set up in such a way it seems that it can't
find our `sha.exs` file.

This is a cleaner approach that doesn't rely on using external files for
the application version.

Interesting note: `mix compile` will happily use the cached `version`
even though it's computed from an env var, because `mix compile` uses
file hash and mtime to know when to recompile.

See https://github.com/firezone/firezone/network/updates/942719116

elixir/apps/api/mix.exs

@@ -2,11 +2,9 @@ defmodule API.MixProject do
   use Mix.Project
 
   def project do
-    {sha, _} = Code.eval_file(Path.join([__DIR__, "..", "..", "sha.exs"]))
-
     [
       app: :api,
-      version: "0.1.0+#{sha}",
+      version: version(),
       build_path: "../../_build",
       config_path: "../../config/config.exs",
       deps_path: "../../deps",
@@ -79,4 +77,9 @@ defmodule API.MixProject do
       test: ["ecto.create --quiet", "ecto.migrate", "test"]
     ]
   end
+
+  defp version do
+    sha = System.get_env("GIT_SHA", "deadbeef") |> String.trim()
+    "0.1.0+#{sha}"
+  end
 end