Fetch value from file which path is specified in an environment variable.

Newline in the end of file is ignored.

Example

iex> :ok = System.delete_env("SOME_TEST_FILE")
...> :error = Elixir.Confex.Adapters.FileFromSystemEnvironment.fetch_value("SOME_TEST_FILE")
...> :ok = System.put_env("SOME_TEST_FILE", "test/fixtures/file_secret.txt")
...> {:ok, "foo_bar"} = Elixir.Confex.Adapters.FileFromSystemEnvironment.fetch_value("SOME_TEST_FILE")
{:ok, "foo_bar"}