Fix diagnostics issue when config_env is called.

[scottming]

The main reason why the diagnostics of runtime.exs cannot be cleared is because of a runtime error

So we need to set a value for the env key.

%RuntimeError{message: "no :env key was given to this configuration file"}], [file: ~c"lib/lexical/remote_control/build/document/compilers/config.ex", line: 79]}, {LXical.RemoteControl.Build.Document.Compilers.Config, :raw_compile, 1, [file: ~c"lib/lexical/remote_control/build/document/compilers/config.ex", line: 55]}, {Code, :with_diagnostics, 2, [file: ~c"lib/code.ex", line: 574]}, {LXical.RemoteControl.Build.Document.Compilers.Config, :compile_with_diagnostics, 1, [file: ~c"lib/lexical/remote_control/build/document/compilers/config.ex", line: 61]}, {Mix.Project, :in_project, 4, [file: ~c"lib/mix/project.ex", line: 458]}, {File, :cd!, 2, [file: ~c"lib/file.ex", line: 1624]}, {LXical.RemoteControl.Mix, :"-in_project/2-fun-0-", 3, [file: ~c"lib/lexical/remote_control/mix.ex", line: 29]}, {:global, :trans, 4, [file: ~c"global.erl", line: 477]}

https://github.com/elixir-lang/elixir/blob/v1.15.7/lib/elixir/lib/config.ex#L220

Fixes #402

[zachallaun]

The actual change looks innocuous enough. I had one question about the tests.