Should pip --upgrade-strategy only-if-needed be a default argument?

[nh13]

We already mention it in the scala docs here:

conda-env-builder/tools/src/com/github/condaincubator/condaenvbuilder/api/PipStep.scala

Lines 43 to 57 in 38f41ac

	/** Example YAML encoding for [[PipStep]]
	* {{{
	* - pip:
	* args:
	* - --upgrade-strategy
	* - only-if-needed
	* requirements:
	* - defopt==5.1.0
	* - samwell
	* }}}
	*
	* Both `args` and `requirements` are optional.
	*
	* Requirements may omit version numbers, which can be compiled from the default environment
	*/

And it's part of the conda doc recommendations here: https://conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#using-pip-in-an-environment

We'd want to support opting out if we make it the default, which could be inelegant.

Thoughts?