logger: Extract LOG_FILENAME from LOG_FILE
logger: Extract LOG_FILENAME from LOG_FILE

So we don't hardcode the logs filename everywhere.

cozy-labs/cozy-desktop,nono/cozy-desktop,nono/cozy-desktop,cozy-labs/cozy-desktop,nono/cozy-desktop,cozy-labs/cozy-desktop,nono/cozy-desktop,cozy-labs/cozy-desktop