Improve CI Docker cache reuse

[lovasoa]

Summary

• Reuse the Linux test binaries and Playwright binary produced by compile_and_lint instead of recompiling sqlpage in the database and browser-test jobs.
• Keep Windows incremental/local sqlpage artifacts in a PR/ref-scoped cache so source edits can reuse incremental compilation state without allowing PRs to write a trusted main cache.
• Make Docker cache scopes explicit: PR caches write to PR-scoped BuildKit GHA scopes, while trusted main remains a restore-only Docker Hub fallback for PRs.
• Move .cargo/config.toml into the Docker dependency warmup layer so the warmup and final build use the same LIBSQLITE3_FLAGS.
• Keep .git/ out of the Docker context and include all Docker dependency-recipe inputs in the cache scope hash.

Current findings

• The Rust job deduplication works. In CI run 26892512893, the DB matrix and Playwright jobs used uploaded binaries and did not run Cargo builds.
• The Docker final-build warmup gap is fixed. In 26892512893, amd64-minimal build-project.sh compiled only sqlpage; the previous repeat compiles of libsqlite3-sys, sqlx-core-oldapi, sqlx-macros-oldapi, and sqlx-oldapi disappeared after copying .cargo/ before build-dependencies.sh.
• Docker BuildKit type=gha layer reuse is still not enough for the expensive dependency layer. Same-commit rerun 26892512893, attempt 2, imported the recipe-scoped GHA cache but still ran build-dependencies.sh, including crate downloads and a full dependency compile.

Validation

• git diff --check
• actionlint -ignore 'SC2046' .github/workflows/ci.yml
• docker buildx build --check --target builder .
• CI run 26892512893: full workflow passed.
• CI run 26892512893, attempt 2, job 79325419825: same-commit amd64-minimal rerun proved the Docker GHA cache import still misses the dependency layer.

Reverted experiments

• BuildKit Cargo cache mounts were tested and reverted earlier. On same-SHA rerun they still downloaded hundreds of crates and compiled hundreds of units, worse than plain layer caching.
• A dummy lib-target warmup was tested and removed. It did not stop SQLx recompilation; copying .cargo/config.toml before dependency warmup did.