mux: forced package_manager check fails use_cached starts when the package manager is missing #1099

Follow-up from #1095 (review thread #1095 (comment)).

scripts/install.sh.tftpl validates a forced package_manager (command -v "$PM_CMD") right after the offline check, before it looks at the cached binary. With use_cached = true, a valid Mux install and a rebuilt image that no longer ships the configured package manager, the script exits with "Configured package manager '...' not found on PATH" even though use_cached promises to run whatever copy is present without touching the registry.

Proposed change: keep the package-manager detection where it is (the version lookup needs PM_CMD), but move the hard failure for a missing forced package manager to the NEEDS_INSTALL = true branch. When use_cached short-circuits, the check is never reached; when the version lookup cannot resolve because the package manager is missing, the install branch fails with the same message as today. Add a container test: forced package_manager, use_cached = true, binary present, package manager absent, expect the start to succeed.

Xum acted on behalf of @ibetitsmike when filing this issue.