fix: fetch builder image per platform

[ddusht]

Summary

When using pack build --platform linux/<arch> against a Docker daemon with containerd storage (io.containerd.snapshotter.v1), the builder image was fetched with Fetch() which returns the manifest list or the host-arch image config. This caused two failures depending on Docker version:

• Docker 28.x + containerd: NotFound: content digest
• Docker 29.x + containerd: missing label 'io.buildpacks.builder.metadata'

Both share the same root cause: pack did not scope the builder image config lookup to the requested --platform.

This PR switches the builder image fetch from Fetch() to FetchForPlatform(), which resolves the platform-specific digest from the registry manifest list before pulling — the same fix already applied to lifecycle image fetching (#2467).

Also tracks FetchForPlatformCalls separately in FakeImageFetcher and adds regression tests.

Output

Before

ERROR: failed to build: invalid builder '<builder>': builder '<builder>'
missing label 'io.buildpacks.builder.metadata' -- try recreating builder

ERROR: failed to build: failed to fetch base layers: saving image with ID
"sha256:9664dab..." from the docker daemon: Error response from daemon:
unable to create manifests file: NotFound: content digest sha256:508c4e1...: not found

After

Build proceeds normally when --platform linux/arm64 is specified against a containerd-backed daemon.

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

Resolves #2587
Resolves #2588

[ddusht]

@jjbustamante is there a way to get this pr prioritized?
this has been a blocker for our company to enable multi-arch support