Remove build workaround for Ruby 4.0.x on Ubuntu-latest

[peteryates]

Introduced in #606 - remove once it's fixed in either Bundler or the Ubuntu image used by GitHub actions.

govuk-form-builder/.github/workflows/tests.yml

Lines 20 to 26 in 2faf79a

	# Workaround: The ubuntu-latest runner image (20260126.10+) ships a pre-installed Ruby 4.0.1
	# that causes native extension builds and bundler operations to fail. Removing it forces
	# setup-ruby to download a working build from ruby-builder instead.
	# See https://github.com/ruby/rubygems/issues/9284.
	- name: Remove pre-installed Ruby 4.0
	if: matrix.ruby == '4.0.1'
	run: rm -rf /opt/hostedtoolcache/Ruby/4.0*