Special `EDITOR` variable for sops

[musjj]

A special EDITOR variable for sops would be nice to have (e.g. SOPS_EDITOR following the GIT_EDITOR, SUDO_EDITOR, etc. convention). My use case is for better integration with GUI editors.

For example, I have my EDITOR set to code because I don't want to block my terminal when opening files in VS Code. But for GIT_EDITOR, I set it to code -rw because I want it to block so that git waits until I'm done writing my commit message.

[felixfontein]

Sounds like a good idea to me!

[felixfontein]

I created #1611 to implement this.