tests: Update cache dir test to not build different distro

[cpuguy83]

This test was always building a distro that is not the current distro under test for the purpose of validating cache key generation. This is undesirable for CI because we want to split each distro into separate jobs.
Before this change the the job for, e.g., testing jammy would also build noble.

This change adds a build tag that we use for tests that injects the same distro implementation twice with but with a modified target key name. e.g. adds "jammy" and "jammyalt".
This allows us to test the same case but without having to build an entirely different distro stack.

[Copilot]

Pull request overview

This pull request refactors the cache directory test to avoid building multiple distros within a single test, enabling better CI job parallelization by allowing each distro to be tested in separate jobs.

Changes:

• Introduces a build tag mechanism (alt_testing_targets) that conditionally registers duplicate distro implementations with an "alt" suffix (e.g., "jammy" and "jammyalt")
• Modifies the Dockerfile to accept an EXTRA_BUILD_FLAGS ARG for passing build tags
• Updates the test infrastructure to build the frontend with the alt_testing_targets tag enabled
• Refactors the cache test to use the alt target instead of building a completely different distro

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Dockerfile	Adds EXTRA_BUILD_FLAGS ARG and applies it to the go build command for the frontend
test/testenv/build.go	Passes -tags alt_testing_targets via build-arg when building the frontend for tests
internal/frontendapi/router.go	Adds logic to register alt target keys when the build tag is enabled, adds AltTargetKey helper function
internal/frontendapi/alt_targets.go	New file with build tag constraint that enables alt testing targets
internal/frontendapi/no_alt_targets.go	New file with build tag constraint that disables alt testing targets by default
test/cache_test.go	Refactors checkDistro function to use alt target instead of hardcoded different distro, removes unused path import

[invidian]

This PR seems like a lot of hacking and custom code just to support a single test. Do you think this indicates a bigger problem with code which would need a better long-term solution? Maybe it would be better to turn off this test temporarily until we have a better solution? Or maybe this specific test could be moved to the Other suite, rather than running for every distro?

Unfortunately, as a still fresh person to the project, I am not able to propose an alternative solution at the moment, but I would be happy to look for a better solution.

[cpuguy83]

I do think we need to look at how we setup each distro. That whole abstraction needs an overhaul.
Though a lot of changes are happening in that area in general right now.