The cache key included only os and arch (`mise-v1-macos-arm64-<hash>`),
so any repo running CI on multiple runner providers with the same
os/arch — github-hosted, namespace.so, BuildJet, self-hosted M-series
macs — would clobber each other's caches. The first run on a new
provider would restore tool installs built for the previous provider's
image, leading to "does not have an executable named X" errors or
SIGILL crashes on cached binaries built against a different glibc.
Append the GitHub Actions hosted-runner ImageOS env var (e.g.
"macos15", "ubuntu24") to the cache key's platform segment. Other
runners pool under "self-hosted"; users running multiple self-hosted
profiles can scope further with cache_key_prefix.
Implementation note: `getTarget()` is also used to construct the mise
binary download URL (must match the release-asset filename, e.g.
`mise-v2026.4.0-macos-arm64.tar.gz`). Adding the image suffix there
would 404 the download. So `getTarget()` keeps its current shape and
a new `getRunnerImageId()` helper composes the image discriminator
into the cache-key platform segment only.
This is a one-time cache miss for existing users; the cache rebuilds
on the next run and stays scoped per-image after that.
