fix(ci): let main warm research compiler cache

[proerror77]

Change contract

Let the Research image binaries job write new sccache objects only on main; keep pull-request runs read-only. This preserves the 45-minute cold-cache headroom while allowing the shared main cache to learn from compiler misses.

Issue relationship

None

Out of scope

No collector, uploader, data, credential, ACR, or live-runtime change. Other Rust CI jobs retain their existing cache behavior.

Dependencies and merge order

Depends on #797, already merged as e60c44ab850795b72298432046eba37e7b115571.

Focused validation

• Regression assertion failed before the workflow change and passed after it.
• .github/scripts/test-select-rust-ci-scope.sh passed.
• git diff --check and shell syntax checks passed.
• actionlint is not installed locally; GitHub workflow lint remains the authoritative check.

Rollout and rollback

After merge, verify an exact main run records READ_WRITE behavior and persists misses without timeout; verify the next main build restores and uses the warmed cache. Revert this PR if main cache persistence again causes a timeout.

Scope exception

None

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 092d6aa4-eb6f-4a45-b55e-4940d3731870

📥 Commits

Reviewing files that changed from the base of the PR and between e60c44a and 7581b16.

📒 Files selected for processing (2)

• .github/scripts/test-select-rust-ci-scope.sh
• .github/workflows/ploy-ci.yml

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