TOML configuration of set_env should also support loading from environment files

[brianhelba]

What's the problem this feature will solve?

Loading environment files in set_env was implemented by @gaborbernat in #1668 and is currently documented as being generally usable. However, it appears that the file| syntax and custom parsing code is incompatible with a TOML configuration of set_env.

Describe the solution you'd like

TOML configuration of set_env should support the ability to load environment files. This would probably be most elegant with some new syntax, as the file| syntax doesn't map cleanly onto the expected type of set_env in TOML.

Alternative Solutions

Alternatively, the documentation should be updated to clarify that the environment file loading / file| syntax is only supported in the legacy INI configuration file format.

[gaborbernat]

PR welcome 👍

[juditnovak]

I made an attempt to address the issue in #3478 . Please let me know what you think.