[ENH] - Implement log flushing

[nkaretnikov]

Feature description

When an env is being built via build_conda_environment, several actions are running. Each of these actions accumulates output into a StringIO object in ActionContext, which is later written to log storage.

Instead, we need to incrementally write to log storage as soon as the output is available, so that users could get visual indication of progress while the job is running.

See #806 (comment).

Value and/or benefit

Improved UX.

Anything else?

• Moving logged_command to run (in action)
• Note: make sure log file is not getting overwritten when you repeatedly write to storage (locking)