Allow if-not-present pull policy for builds with image extensions

[MoritzWeber0]

Description

When running pack build and the builder contains image extensions, the build fails with:

ERROR: failed to build: pull policy must be 'always' when builder contains image extensions

Proposed solution

The pack build command should accept --pull-policy if-not-present as argument, also if the builder contains image extensions.

Additional context

I don't want to rely on a local registry or internet connection when building my images locally.

[jjbustamante]

Make sense to me, but no sure if @natalieparellano has some thoughts about it

[natalieparellano]

The issue with --pull-policy if-not-present is that we will not get a manifest for an image that only exists in the daemon (though with newer versions of docker using containerd as the backing store, that will hopefully one day be possible).

However, I believe some recent changes to imgutil (see https://github.com/buildpacks/imgutil/blob/main/local/v1_facade.go) should make it possible to construct a dummy manifest that may be good enough for our purposes (kaniko). We should do an exploration for it.