Without a per-arch SCCACHE_GHA_VERSION, both matrix jobs
(linux-amd64-cpu8 and linux-arm64-cpu8) target the same GHA cache
key. Run 1 on 2026-04-24 showed this concretely: arm64 started
first and landed 575/1064 cache writes; amd64 started ~1 min later
and every one of its 1062 writes failed, leaving the cache with no
amd64 entries at all.
Partitioning by matrix.runner ensures each arch gets its own
cache namespace, letting us see real warm-cache behavior on the
subsequent Run 2-4 dispatches.
Signed-off-by: Jonas Toelke <jtoelke@nvidia.com>
jtoelke2