Environment from command line updated twice

[psss]

Seems that in a76772f we've introduced duplicate plan environment update from the command line options.

The first is here:

tmt/tmt/base.py

Lines 1558 to 1562 in 722563c

	self._environment = tmt.utils.environment_from_spec(
	raw_fmf_environment_files=node.get("environment-file") or [],
	raw_fmf_environment=node.get('environment', {}),
	raw_cli_environment_files=self.opt('environment-file') or [],
	raw_cli_environment=self.opt('environment'),

And the second one here:

tmt/tmt/base.py

Lines 1676 to 1677 in 722563c

	# Command line variables take precedence
	combined.update(self.my_run.environment)

Seems it does not break anything but we should get rid of the duplication.

[psss]

Summary from the hacking session discussion:

• move the gathering of imported plan environment to an environment property
• make sure self._environment is not being accessed directly in tmt code (property should be used)