Use env shebangs for RubyGems installs

[joshheinrichs-shopify]

Motivation

RubyGems bin wrappers hardcode the Ruby used at install time. With Nix-style Ruby installs, that path can later be garbage-collected or replaced, leaving wrappers like ruby-lsp and bundle pointing at a stale store path.

Implementation

Ruby LSP already activates the project Ruby environment before installing or launching the server, so install the bootstrap ruby-lsp gem and any missing Bundler versions with RubyGems' env-shebang option. This lets generated wrappers resolve ruby from the activated PATH instead.

Bundler project binstubs already use env shebangs by default; this makes the RubyGems bin wrappers Ruby LSP relies on behave similarly.

Automated Tests

Yes.

Manual Tests

In a clean project, ruby-lsp and bundler should have #!/usr/bin/env ruby as the shebang.

[pcasaretto]

Thanks for the fix, Josh. I tested this branch locally and this looks good to me.

Validation performed:

• Ruby-side regression coverage passes: test/setup_bundler_test.rb completed with 41 runs, 189 assertions, and 0 failures/errors when run in a Nix shell with libyaml/pkg-config available for psych.
• VS Code extension code compiles successfully.
• The PR-specific VS Code assertions for installing/updating ruby-lsp with --env-shebang pass. I had to run from a short /tmp worktree because the deeper macOS path hit the Unix socket path length limit in vscode-test; unrelated Chruby/debugger environment tests still failed in that local setup, but the assertions added by this PR passed.
• Manual RubyGems verification confirms the generated ruby-lsp wrapper starts with #!/usr/bin/env ruby when installed with --env-shebang.

This addresses the Nix/Ruby refresh failure mode we saw where generated binstubs captured an absolute stale Nix store Ruby path. The main boundary to keep in mind is that this prevents newly installed/updated wrappers from going stale; existing stale binstubs still need to be regenerated by an install/update/pristine path. That is an acceptable scope for this PR.