ci: remove coverage instrumentation from test builds

[huth-stacks]

What

Remove -Cinstrument-coverage from RUSTFLAGS in the test cache build.

Why

Coverage instrumentation is applied to ALL test binaries via create-cache.yml. This adds:

• ~15% compilation overhead to the 15-19 minute nextest archive build
• ~56% binary size increase (43MB → 67MB), slowing archive transfer to 100+ test runners
• .profraw file I/O on every test execution
• grcov processing time on every test job

Coverage data is only consumed by the trigger-code-coverage-report job. For PR feedback, tests passing/failing matters more than coverage diffs.

The Change

In .github/workflows/create-cache.yml:

-  RUSTFLAGS: "-Cinstrument-coverage -Awarnings"
+  RUSTFLAGS: "-Awarnings"

1 line changed in 1 file.

Metrics to Track

Metric	Baseline	Expected
nextest-archive build	15-19m	~13-16m (-15%)
Archive binary size	~67MB	~43MB (-36%)
Coverage report	Runs	Skips gracefully

Risk

• Coverage diffs no longer reported per-PR on Coveralls
• Coverage can be re-enabled per-merge or restored by reverting this line
• David Haney's PR Updated ci.yml to have a 'v2' of code coverage action, which is more robust, is skipped when prior steps are skipped, and uses env vars instead of hard-coding stacks-network/stacks-core#7031 (coverage report improvements) is orthogonal — he modifies report merging in ci.yml, we modify instrumentation in create-cache.yml

Security Checklist

• No new permissions
• No secrets exposure
• RUSTFLAGS change is purely subtractive

Part of CI Optimization Series

PR 9. Expected impact: ~2-3 minutes on archive build + reduced test runner overhead.