Optimize CI

[filip-parity]

Cache fix - biggest improvement here. We were saving caches for each PR run (~2.5GB per platform) which were replacing the master cache due to the 10GB
limit, so no PR was ever getting actual cache hits => full rebuild every time. Removed saving caches on PRs so the master cache (from daily test-isolate
cron) stays available for everyone. Also made crate-checks cache read-only for the same reason.

Anvil-polkadot to linux-only + release mode - these tests were taking 80-115 min on macOS/Windows. Moved to linux-only with --release, finishes in ~19
min now.

Test matrix changes - dropped partitions from 2-3 to 1 for integration, issue-repros, and external. With a warm cache these finish fast enough on their
own. This also fixes the issue where too many parallel jobs compete for runners and get cancelled mid-execution. PR jobs: 14 → 9.

Mold linker - added to all Linux jobs for faster linking.

[pkhry]

WRT anvil-polkadot slow execution times: How about we move it to a separate linux-only worker completely and run it with modified config (a la test external projects, release only mode with some debug settings on) that way we have to compile way less(only anvil-polkadot in release mode and it's deps, no caching is needed) and the job should finish in around ~20 mins? cc @re-gius @smiasojed. so basically just removing windows and macos tests for it right now and configuring compilation settings differently

[re-gius]

WRT anvil-polkadot slow execution times: How about we move it to a separate linux-only worker completely and run it with modified config (a la test external projects, release only mode with some debug settings on)

I agree: we can only keep linux tests for anvil-polkadot in CI + compile in release mode.

[pkhry]

do not use --release, define a profile that inherits from release but adds debug info