Add Ubuntu 26.04 support

[enescakir]

GitHub recently launched Ubuntu 26.04 support in public preview.

actions/runner-images#14226

Currently it fails with following:

Error: The current runner (ubuntu-26.04-x64) was detected as self-hosted because the platform does not match a GitHub-hosted runner image (or that image is deprecated and no longer supported).

[eregon]

Starting PR: ruby/ruby-builder#29