Summary

Add a CI job that reproduces the fresh-install condition behind #122 — an environment with no
baseruby on PATH — so the ruby source-build deadlock is structurally guarded against regression.

Background / Why

#122 fixed the deadlock by setting ruby.compile = false (precompiled ruby) in the mise config.
However, the deadlock only manifests when there is no usable ruby on PATH: ruby-build's configure
probe re-enters the mise ruby shim for the in-progress version and blocks on the install lock.

Standard CI runners (macOS, Ubuntu) already ship a system ruby, so they never reproduce the
deadlock. As a result, the current setup-validation workflow cannot catch a regression if
ruby.compile = false is ever removed or overridden. The fix is therefore unguarded in CI.

Acceptance criteria

• CI exercises a mise install in an environment with no baseruby on PATH, and fails fast
  (no indefinite hang) if precompiled ruby is not enforced.
• The job passes on the current config (ruby.compile = false) and is wired into the existing
  validation workflow.

Related

• Parent: Make mise tool installation robust on fresh chezmoi apply #123
• Fixes the unguarded case from mise ruby source-build deadlocks without baseruby on fresh apply #122 (PR fix(mise): use precompiled ruby to avoid source-build deadlock (#122) #125 )