Improves CI performance

[NickSdot]

tl/dr: ~45m -> ~22m; this is CI-only stuff -- everything in #22917 reduces further.

Results

f47a44e (perf: speed up Windows CI)

Windows x64	Before	After	Improvement
Setup + build	10:53	3:22	69%
Tests	30:37	14:48	52%
Full job	41:55	18:48	55%

Cache hit rate: ~95%.

8e526f1 (perf: speed up benchmark checkout)

Avoids downloading unnecessary file contents. From ~1:11 to ~0:20, which is a 72% improvement.

653fa83 (perf: keep non-Windows compiler caches fresh)

Before: the older the weekly cache, the lower the hit rate. 2026-07-27 was ~99% (0:30s) while 2026-08-02 was only ~25% (6:52). Rolling cache solves that.

Compile job	Before (1)	After	Improvement
Linux release	7:54	0:28	94%
Benchmark	2:33	0:17	89%
Unit tests	2:06	0:14	89%
Linux ASAN	1:50	0:27	75%
macOS	4:07	1:30	64%
Alpine ASAN	2:37	0:57	64%
Linux x32	2:10	0:57	56%

Cache hit rate: ~99%.
(1) when cache not fresh.

6c933cb (perf: skip unused ASAN setup)

Setup	Before	After	Improvement
Service containers	1:24	0:13	85%
MSSQL setup	0:14	Skipped	100%
Apt dependencies	0:54	0:42	22%
Total targeted setup	2:32	0:55	64%

Saves 1:37 per ASAN run. Depending on download speeds shaves off 1–3 min.

a33ae4c (perf: run Function JIT tests concurrently)

Splits from sequentially to separate jobs for normal (16:37m) and JIT (18:47 ); results in 11 minutes earlier finish.

3dcc018 (perf: shard long running jobs)

Critical path dropped from 44:27 to 22:49 (49%). Slowest shards finished in 19:03 for Linux ASAN, 17:39 for Windows x64 ZTS, and 14:43 for Alpine ASAN.

58c43d5 (perf: aded circle ci compile caching)

CircleCI	Before	Warm cache	Improvement
Compile	8:17	3:23	59%

Cache hit rate: ~99%

Warm cache compile down from ~8:17 to ~3:23, which is 59% improvement. Works slightly different than the other caching. Circle isolates caches that belong to external forks. Trusted master builds roll the source repository cache forward; same-repository PRs can restore it, fork PRs start cold and then roll their own forward.

[NickSdot]

@arnaud-lb maybe it is worth prioritising this one before the others. Would give me a much faster feedback loop for all the rest that likely will receive some more pushes if there is feedback from you to address. Will you look at this one too or should I mark as ready to ping the reviewers?