ci: Wait for nixbot before building the Nix matrix

[brainrake]

What kind of change does this PR introduce?

CI improvement.

What is the current behavior?

Nix CI builds the full matrix on GitHub runners, even when nixbot is building the same commit.

What is the new behavior?

A new await-nixbot job waits for the nixbot/nix-eval and nixbot/nix-build check runs to finish before eval runs. The matrix then only builds what is missing from the cache. If nixbot posts no check run within 1 minute, CI runs the matrix as before.

Additional context

eval runs even if the wait job fails or times out.

🤖 Generated with Claude Code

[blacksmith-sh]

Found 1 test failure on Blacksmith runners:

Failure

Test	View Logs
test_ami_nix/test_coredump_processor_produces_diagnostic_bundle_and_deletes_raw_core	View Logs

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