Show current build step while it's executing

[ericholscher]

What's the problem this feature will solve?

Currently, when the build is running, we don't show the current step, so if a step is taking a long time user don't actually know what is going on behind the scenes

Describe the solution you'd like

We should set the current build command when it's started, and then updated it without output when it's finished

Additional context

The code for running and saving the command is here:

readthedocs.org/readthedocs/doc_builder/environments.py

Lines 470 to 472 in fd9aeed

	def run_command_class(
	self, cls, cmd, warn_only=False, record=True, record_as_success=False, **kwargs
	):

-- we likely just need to update the logic to save the command name at the start, store the command ID, and then update it instead of create it with it's done.

[humitos]

Closing as duplicate of #4288

We also have some internal discussion about this at https://github.com/readthedocs/meta/discussions/141