Use clean environment when generating test eggs

[mdickinson]

For the egg-based tests, we should use a clean environment when creating test eggs. There have been reports of stray environment variables causing issues with running the test suite on Debian. (See #499.)

Note that a perfectly clean environment doesn't seem to work well on Windows and EDM; there are likely at least some environment variables that need to be carried over from the parent environment.

[mdickinson]

Just a note that the plan is to deprecate the EggPluginManager and EggBasketPluginManager in the next release of Envisage (7.0). Once these are removed in Envisage 8.0, the hard-to-maintain machinery for egg building can go away.