Use fid+signer provided in .env

[vrypan]

During development, it's easy to generate a signer for my own fid.
(https://github.com/vrypan/fario/blob/main/HOWTO/Create_and_use_Signers_with_fario.md#create-a-signer)

It would be great if I can add them to my .env and have frog use them during testing.

[tmm]

Good idea. Will add this to my todo list!