Enable unit tests on Ubuntu 26.04 once a GitHub Actions runner image is available for it

[edmorley]

The buildpack has some unit tests for cgroups support that run directly on the GitHub Actions runner images, here:

heroku-buildpack-php/.github/workflows/ci.yml

Lines 33 to 48 in c9b2a5c

	unit-test:
	runs-on: ${{ matrix.os }}
	strategy:
	fail-fast: false
	matrix:
	os: ["ubuntu-22.04", "ubuntu-24.04"]
	steps:
	- name: Checkout
	uses: actions/checkout@v6
	- name: Install Ruby and Bundler
	uses: ruby/setup-ruby@v1
	with:
	bundler-cache: true
	ruby-version: "${{ env.setup_ruby_ruby_version }}"
	- name: Execute tests
	run: bundle exec rspec test/spec/cgroup_spec.rb

There isn't yet a GitHub Actions ubuntu-26.04 runner image, so we won't be able to enable these tests as part of the initial PR to add Heroku-26 support.

However, once the runner image is available upstream we should enable them.

https://docs.github.com/en/actions/reference/runners/github-hosted-runners#standard-github-hosted-runners-for-public-repositories