CI: github: fix VTest cache paths in container jobs

CI: github: fix VTest cache paths in container jobs

The Alpine job rebuilds VTest without publishing a cache entry. Its
cache save receives the host workspace path, which does not exist inside
the container, and reports a Path Validation Error. Changing only the
cache path to "vtest" still leaves installation using the context path,
so the cached directory remains empty.

Install VTest under the runtime GITHUB_WORKSPACE and use the same
location in all workflows that call setup-vtest. Keep restore and save
paths workspace-relative so that the installed binary is found inside
the container and cache hits restore it where the callers expect it.

Split caching into explicit restore and save steps, saving immediately
after installing VTest rather than relying on the post-job hook. Reuse
the restore step's primary key. Include the distribution and version in
the key to keep Alpine/musl binaries separate from glibc builds on
runners with the same architecture.