End to end and integration tests should be run for a controlled matrix of compiler releases

[euanh]

The end to end and integration tests use the Swift compiler provided by the GitHub CI runner. The runner image was updated to include Swift 6.1 (https://github.com/actions/runner-images/blob/ubuntu24/20250406.1/images/ubuntu/Ubuntu2404-Readme.md) but this caused the end to end and integration tests to fail because they install the 6.0.2 release of the Static Linux SDK, which is not compatible with Swift 6.1.

#79 fixes this problem in the short term by updating the workflows to a compatible version of the Static Linux SDK, but ideally these tests should not depend on a the particular version of Swift in a runner image, and should also test multiple versions.

These tests do not run in containers because part of the test involves building and running a container image, which is inconvenient when the test itself runs in a container, as used for the matrix unit tests.

Possible solutions:

• run the end to end and integration tests in privileged containers and pass the Docker socket through so images can be built and run
• run the tests in Docker-in-Docker, which can make it easier to set up and tear down such recursive containers.
• Use named runner images with known Swift compilers instead of ubuntu-latest.
• Install specific compilers using swiftly.