ci: make BuildKit registry cache optional in component-build

[BrianEstrada]

Summary

• Adds an enable-buildkit-cache boolean input (default true) to component-build.yml so callers can opt out of the registry-backed BuildKit cache.
• When false, both cache-from and cache-to resolve to empty strings and docker/build-push-action skips the registry cache entirely. Default behavior is unchanged for existing callers.

Test plan

• Trigger a downstream build with enable-buildkit-cache: false and confirm no buildcache-* tag is read or written
• Trigger a downstream build with the default (or true) and confirm cache hit/export still works as before

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