fix: use SF env var in loglevel msg

[cristiand391]

updates --loglevel msg to mention SF style env var instead of the SFDX ones, no functional change since sfdx-core supports both

➜  jsforce git:(main) ✗ sf org display --loglevel DEBUG
Warning: The loglevel flag is no longer in use on this command. You may use it without error, but it will be ignored.
Set the log level using the `SFDX_LOG_LEVEL` environment variable.

[skip-validate-pr]