run cargo audit with --deny-warnings enabled

[lauralt]

Without adding this flag, breaking changes related to cargo audit (which may turn into errors in time, see this thread) won't be caught by test_sec_audit.py because we are not passing --deny-warnings flag to cargo audit. By passing the flag, the test will fail because of the warning that will be emitted when such changes are happening. See this comment.

To replicate the error, on v0.21.3 or v0.22.1, it's enough to just run the test_sec_audit.py.
Relevant line in output:

E error: error loading advisory database: parse error: unexpected character found: ``` at line 1 column 1.

It is also worth mentioning, if we choose to run cargo audit with --deny-warnings, using yanked versions won't be allowed anymore (versionize and versionize_derive at this moment). To check this, you can run on the current master (commit 176e8e8) test_sec_audit.py, after updating this line to:

utils.run_cmd('cargo audit --deny-warnings -q -f {}'.format(cargo_lock_path)).

(Or simply just run in firecracker cargo audit --deny-warnings -q -f Cargo.lock)

[alxiord]

Blocked since firecracker-microvm/versionize#23 (included in versionize v0.1.4)

[dianpopa]

Fixed by #2858