Change RUST_LOG variable name

[gdorsi44]

Hello Spin team,

Is it a good move consider the possibility to modify the log variable RUST_LOG with sort of like SPIN_LOG for avoid misconfusions during a scenario with multiple runtimes/languages laid down in a project?

[michelleN]

Linking #608 and #721 here as it's probably related to the overall story around debugging and verbosity.

[gdorsi44]

Linking #608 and #721 here as it's probably related to the overall story around debugging and verbosity.

Thank you @michelleN