Support running all external commands under a wrapper

[acmorrow]

Describe the Feature
Per the linked scons-users discussion, none of SPAWN, PSPAWN, or SHELL quite achieves the ability to cleanly inject a wrapper under which all external commands should run. This is required when using icecream, for instance, where all commands should be prefixed with icecrun.

It is also important that this customization honor ordinary subst rules with access to the target and source (so that it can be set to a generator function which can behave intelligently), and that the resulting action string can be obtained so that when this customization is used in a generator context (see #3642), that the wrapper command is included in the generated command string.

Required information

• Link to SCons Users thread discussing your issue.
  https://pairlist4.pair.net/pipermail/scons-users/2020-May/008130.html

• Version of SCons
  N/A: Feature request