[RubyGems] Ignore specs missing extensions

[segiddins]

Closes #5210 but I'm not entirely convinced that this is necessary

[indirect]

@segiddins necessary in what sense? (Also, do you have a sense of the performance impact of this change on an app with 500 gems?)

[segiddins]

Necessary in the sense of requiring the extension will still work if its installed globally, or fail otherwise, even without this change. And no, didn't profile it yet. Keep in mind that it can short-circuit in an RG with stubs for gems that don't include extensions

[segiddins]

This is green, fyi

[indirect]

@bundlerbot r+

[bundlerbot]

📌 Commit 1f10c7f has been approved by indirect

[bundlerbot]

⚡ Test exempted - status

[kirs]

Thanks @segiddins!