blockifier_test_utils: fix ETXTBSY race in cairo package download

[avi-starkware]

verify_cairo1_package double-checks cairo1_package_exists — once
without the lock (fast path) and once after acquiring it. The predicate
only tested .exists() on the two binaries, so a concurrent process
could observe the package as "ready" while tar was still mid-write:

1. Process A holds the download lock, tar is extracting the archive.
2. tar extracts sequentially: first binary is closed, second binary
   is created (so .exists() is true) and being written to.
3. Process B calls cairo1_package_exists, sees both files, skips the
   lock.
4. Process B forks + execs the second binary → Linux returns
   ETXTBSY because the file is still open for writing by tar.

Fix with the same commit-marker pattern used in compile_cache.rs:
write .download_complete after tar finishes, and require it in
cairo1_package_exists. Until the marker exists, concurrent callers
fall through to the lock and wait for the writer.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[reviewable-StarkWare]

This change is 

[cursor]

PR Summary

Low Risk
Low risk: adds a simple completion marker check to avoid concurrent processes executing partially extracted binaries; behavior change is limited to test-utils package download flow.

Overview
Fixes a race in Cairo1 compiler package setup where concurrent callers could treat the package as ready while tar was still writing binaries, leading to ETXTBSY on exec.

The download now writes a .download_complete marker after extraction finishes, and cairo1_package_exists requires this marker in addition to the compiler binaries so the fast-path only returns true for fully extracted packages.

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[dorimedini-starkware]

@dorimedini-starkware reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on avi-starkware).

[dorimedini-starkware]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on avi-starkware).