DRIVERS-2328: Skip installing the legacy shell by default

[alcaeus]

DRIVERS-2328

This PR removes the current SKIP_LEGACY_SHELL environment variable and introduces a new INSTALL_LEGACY_SHELL variable to essentially flip the default behaviour.

Recent work in the Go driver provides a template for drivers to remove usage of the legacy shell. The "Downstream Changes Summary" of the above ticket reads as follows:

The legacy shell will no longer be installed in the near future. Drivers are encouraged to remove all usages of the legacy shell in their drivers, with this Go driver PR serving as inspiration.

All drivers that require the use of the legacy library are required to define the INSTALL_LEGACY_SHELL environment variable to a non-zero value when running MongoDB installation (either by running run-orchestration.sh or by manually invoking download_and_extract() from download-mongoldb.sh). This variable replaces the current SKIP_LEGACY_SHELL variable which will no longer be respected.

If this approach sounds good, I would send the drivers ticket to implementation and add a timeline for this change to be merged. I would propose one month for this, as the change would allow drivers to keep using the legacy shell by adding INSTALL_LEGACY_SHELL=1 to appropriate places before taking the time to remove legacy shell usage.

[blink1073]

LGTM!

[jmikola]

@alcaeus: Should this still be in a draft state?

[alcaeus]

@jmikola yes. I've confirmed that the changes work, but before we merge this we should give downstream teams a heads up and a bit of time to prepare for the change, as merging this will immediately break builds that still rely on the legacy shell. I've moved the DRIVERS ticket to "Teams Implementing", so teams should be ready to pick this up in the next couple of weeks. Until then, I'll leave this PR in a draft state so it doesn't get merged by accident.