Expose rich environment variables to tests

[MatthewMckee4]

Set useful environment variables that tests can read during execution:

• KARVA_RUN_ID — unique identifier for this test run
• KARVA_PROFILE — active profile name
• KARVA_WORKSPACE_ROOT — project root path
• KARVA_TEST_THREADS — configured worker count
• KARVA_VERSION — karva version
• KARVA_ATTEMPT — current retry attempt (1-indexed)

These help tests adapt their behavior (e.g. different logging in CI, unique temp dirs per run) without needing custom fixtures.

See: https://nexte.st/docs/configuration/env-vars/#environment-variables-nextest-sets