Testnet deployments don't work with env variable private keys

[Pet3ris]

Hi There,

When a ~/.ethereum/<network> private key file is not provided for a given network, the alternative environment variable provision may not work.

Tested using the script: https://github.com/compound-finance/compound-protocol/blob/master/spec/sim/0005-grants/hypothetical_upgrade.scen and seeing output:

Running script: spec/sim/0005-grants/deploy.scen...
Command: PrintTransactionLogs
Command: ComptrollerImpl Deploy Standard StdComptrollerG6
EventProcessingError: From required but not given for ComptrollerImpl. Please set a default alias or open unlocked account

I've tried:

• creating an .env file in the root project directory with ACCOUNT=XXX
• adding it like so ACCOUNT=XXX yarn ...