The builder action restore-keys is `flatpak-builder-${arch}`. Our key
was `flatpak-builder-${hash}`, which the action stores as
`flatpak-builder-${hash}-x86_64`. That does not start with
`flatpak-builder-x86_64`, so a packaging hash change (a lock refresh,
or merging one) discarded the whole .flatpak-builder cache and
recompiled numpy and pywhispercpp from source, ~18 min instead of ~4.
Put ${{ matrix.arch }} in the key first so the fallback matches. Then
flatpak-builder skips unchanged modules and only rebuilds what the
hash change actually touched.
PRs no longer watch src/**. python-test already covers those, and the
expensive compile does not. push to main still does, so a sandbox
install break fails the default branch.
