ci: skip Gemfile.lock BUNDLED WITH on ruby-head

[soutaro]

Summary

• The Ruby / Ruby on Windows / C99_compile / clang_compile workflows have been failing on ruby-head (and the ucrt / mswin dev builds on Windows) with NameError: private constant Pathname::SEPARATOR_PAT referenced raised from Bundler::Source::Path#generate_bin.
• Root cause: Pathname::SEPARATOR_PAT became a private constant on ruby-head (4.1.0.dev). Every released bundler up to 4.0.11 still references it. Because Gemfile.lock pins BUNDLED WITH 4.0.1, the bundler 4.1.0.dev that ships with ruby-head downgrades itself to 4.0.1 before running and then crashes.
• The upstream fix is Use Pathname#absolute? rubygems#9529 ("Use Pathname#absolute?", merged 2026-05-07) — but it has not landed in any released bundler yet, so simply bumping BUNDLED WITH would not help.
• This PR adds bundle config set --local version system for ruby-head / ucrt / mswin only. That tells bundler to use the system-installed bundler instead of the one pinned in Gemfile.lock, avoiding the forced downgrade to 4.0.1. Stable Ruby versions are unaffected because Gemfile.lock is untouched.

Once a fixed bundler (4.0.12 or 4.1.0) is released, run bundle update --bundler to bump BUNDLED WITH in both Gemfile.lock and steep/Gemfile.lock, and revert these workaround steps.

Test plan

• Ruby workflow: test (head, ...) jobs go green
• Ruby workflow: C99_compile (head) goes green
• Ruby workflow: clang_compile (head) goes green
• Ruby on Windows workflow: compile (ucrt) and compile (mswin) go green
• No regression on stable Ruby versions (3.2 / 3.3 / 3.4 / 4.0)

🤖 Generated with Claude Code