cache: enable gha cache backend if cache service v2 detected

[crazy-max]

After testing latest RC, gha cache backend is always disabled when cache service v2 is detected. We forgot to check if url_v2 is set in #2983.

Also fixes gha cache url handling to always set both url and url_v2 if possible to match fallback of github actions tookit.

[crazy-max]

Hum it fails to solve in our e2e test with: https://github.com/docker/buildx/actions/runs/13375533199/job/37353936009?pr=3001#step:9:388

#6 [xx internal] load metadata for docker.io/library/alpine:latest
#6 DONE 0.7s
ERROR: target ld64-static-tgz: failed to solve: failed to configure gha cache exporter: url not set for github actions cache

I think that's because we only set url_v2 but the gha cache exporter requires url to be set: https://github.com/moby/buildkit/blob/de39e457a2c91179913c2eb303543f240ca793b3/cache/remotecache/gha/gha.go#L66-L69

So we need a fix on BuildKit as well.

[crazy-max]

Hum it fails to solve in our e2e test with: https://github.com/docker/buildx/actions/runs/13375533199/job/37353936009?pr=3001#step:9:388

In this case it fails because we are pushing an empty url attribute against previous BuildKit version (0.19.0) that doesn't support url_v2. In such case we should always set the url attribute.

Looking at github toolkit source code https://github.com/actions/toolkit/blob/2b08dc18f261b9fdd978b70279b85cbef81af8bc/packages/cache/src/internal/config.ts#L28-L33 they set either ACTIONS_CACHE_URL or ACTIONS_RESULTS_URL if found. Although I doubt ACTIONS_RESULTS_URL would work with v1 🤔

I think we should do the same in buildctl https://github.com/moby/buildkit/blob/de39e457a2c91179913c2eb303543f240ca793b3/cmd/buildctl/build/util.go#L28-L41

[crazy-max]

I think we should do the same in buildctl https://github.com/moby/buildkit/blob/de39e457a2c91179913c2eb303543f240ca793b3/cmd/buildctl/build/util.go#L28-L41

Done in moby/buildkit#5754