ci: enable cargo nextest for ci jobs

[neil2468]

Current Behaviour

The CI jobs use the default cargo test test runner for Rust tests.

Proposed Changes

Switch to using cargo nextest in CI jobs in order to speed up CI.

The changes should not affect builds on developers' machines; cargo nextest is only used if gradlew test is used and the environemnt variable CI is set to true.

For consistency, all references to the previous version of the ockam-builder Docker image have been updated to the new version (which includes nextest).

Fixes #2924.

Checks

• All commits in this Pull Request are signed.
• 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/contributors repository.

[mrinalwadhwa]

ghcr.io/build-trust/ockam-builder:sha256-e43dd94652096b03cc472a3c709c7335e8b166cab77b7a7b56f88fa38f3d24cc