introduce TART_EXECUTOR_NESTED env var in order to support nested vir…

[javgarr]

Introduce TART_EXECUTOR_NESTED env var in order to support nested virtualization (tart >=2.20.0)

More info:
https://tart.run/faq/#nested-virtualization-support
https://github.com/cirruslabs/tart/releases/tag/2.20.0

[javgarr]

For your review, @edigaryev

This is currently a blocker for us in order to be able to test against android emulators on VMs running on M4 Mac Mini machines (MacOS 15+)

[edigaryev]

Hello @javgarr,

Thank you for your contribution 🙌

We generally strive to expose security-sensitive options as config or prepare arguments, rather than environment variables, whenever possible. This approach provides GitLab Runner administrators greater control over what runs on their hosts, any leaves the rest of the non-sensitive configuration to the CI job writers.

Could you refactor this slightly to make it a --nested argument to prepare stage instead of an environment variable?