Mix edeliver doesn't seem to recognize updates to my mix.exs file

I just updated Elixir on my application and I’m trying to use the command mix edeliver build release --branch="$CURRENT_BRANCH" --auto-version=build-date+build-time+git-branch+git-revision to build a release, but it keeps telling me my mix.exs file specifies the previous version of Elixir.

(Partial) content of mix.exs:

  def project do
      app: :project_name,
      version: version(),
      elixir: "~> 1.13.4",
      elixirc_paths: elixirc_paths(Mix.env()),
      compilers: [:phoenix] ++ Mix.compilers(),
      start_permanent: Mix.env() == :prod,
      aliases: aliases(),
      deps: deps(),
      preferred_cli_env: [
        vcr: :test,
        "vcr.delete": :test,
        "vcr.check": :test,
        "vcr.show": :test

Exact Error Message:

** (Mix) You're trying to run :project_name on Elixir v1.13.4 but it has declared in its mix.exs file it supports only Elixir ~> 1.8.1

Running cat mix.exs confirms I’ve got the right version in the Mix file in the directory where I’m trying to run the build command. I’ve searched my workspace for any additional mix.exs files or any occurrences of 1.8.1, but to no avail.

How do I get the build command to work with the new version of Elixir? What else should I be trying/checking?

Try nuking the _build folder. It’s generally good idea to do so on every Elixir & Erlang updates.