[ci] Pin build shard weights per run to keep worker assignments identical (#37373)

The build workers can restore different weights if they don't restore
the cache at the exact same time. The more time difference, the more
likely they restore different weights which could lead to some bundles
not being built at all (e.g.
https://github.com/react/react/actions/runs/32822851267).



A new job now restores the latest entry once per run and republishes it
as a per-run artifact. The new job sits adds no wall time because it
runs in parallel with `runtime_compiler_node_modules_cache`, which
already gates the build workers and takes about 30 seconds on a cache
hit, while the resolve job does strictly less work (no checkout, no Node
setup, a 5KB cache entry instead of the node_modules restore), so it
finishes first and the build workers start at the same time as before.

Co-authored-by: Claude Code (kimi-k3[1m]) <noreply@anthropic.com>