ci: run doctests after cargo nextest

[neil2468]

Current Behavior

Issue #3033 identified that Rust doctests are not being run by the GitHub CI. This is because...

• ci: enable cargo nextest for ci jobs #2949 updated CI to use cargo nextest instead of cargo test.
• However, nextest does not currently support doctests.
• So, doctests have been silently not running.

Proposed Changes

• Modify CI so cargo test --doc is run after cargo nextest.
• In gradle script, rename environmentCI() to environmentVars() so it makes more sense.

Fixes #3033.

Checks

• All commits in this Pull Request are signed and Verified by Github.
• All commits in this Pull Request follow the Ockam commit message convention.
• I accept the Ockam Community Code of Conduct.
• I have accepted the Ockam Contributor License Agreement by adding my Git/Github details in a row at the end of the CONTRIBUTORS.csv file in a separate pull request to the build-trust/ockam-contributors repository.

[mrinalwadhwa]

Thank you for another awesome contribution! That looks great 🥳