Bug: InputDigest tagger fails with artifact dependency and COPY --from

## Environment
Skaffold version: v2.16.0
OS: MacOS
Docker engine version: 28.2.2

## Description

Since skaffold v2.14.0 https://github.com/GoogleContainerTools/skaffold/releases/tag/v2.14.0 the inputDigest tagger is failing for images that copy from an artifact dependency. The described issue is not reproducible in v2.13.x.

It _may_ have been caused by https://github.com/GoogleContainerTools/skaffold/pull/9664 which was part of that release.

When the inputDigest tagger fails, skaffold falls back to the sha256 builder which results in the image being tagged as latest.

This seems to have been previously fixed by skaffold in https://github.com/GoogleContainerTools/skaffold/pull/5507 but https://github.com/GoogleContainerTools/skaffold/pull/9664 did a refactoring of the code, potentially re-introducing the bug.
Of note however is that reproducing the steps defined in https://github.com/GoogleContainerTools/skaffold/issues/5498 did not result in the problem. It seems this current bug is localised to usage of COPY --from.

Usage of `COPY --from` where _from_ is an image provided by a docker ARG requires the use a named `FROM` due to `--from` not expanding variables (see https://github.com/moby/moby/issues/34482).

The following is what the debug logs output when calculating the image tag.

> DEBU[0013] generating tag: parsing ONBUILD instructions: retrieving image "image:latest": GET https://index.docker.io/v2/library/image/manifests/latest: UNAUTHORIZED: authentication required; [map[Action:pull Class: Name:library/image Type:repository]] subtask=app task=Build
> DEBU[0013] Using a fall-back tagger subtask=app task=Build

## Expected behavior

```
Generating tags...
- app -> app:645bd86c539773a11acd886285d3e527e2cc439caf28a7812aa4c334c3a2b3e4
- base -> base:f06ab6582db5b2f039bbfe557419b897c31a4161c69c6e7c7832f839b021a7e4
Checking cache...
- app: Not found. Building
- base: Not found. Building
Starting build...
Building [base]...
Sending build context to Docker daemon 3.072kB
...
Successfully built e7294fc7e8ea
Successfully tagged base:f06ab6582db5b2f039bbfe557419b897c31a4161c69c6e7c7832f839b021a7e4
Build [base] succeeded
Building [app]...
Sending build context to Docker daemon 4.608kB
...
Successfully built a8ac08d9dedf
Successfully tagged app:645bd86c539773a11acd886285d3e527e2cc439caf28a7812aa4c334c3a2b3e4
Build [app] succeeded
```

## Actual behavior

```
Generating tags...
- app -> app:latest
- base -> base:f06ab6582db5b2f039bbfe557419b897c31a4161c69c6e7c7832f839b021a7e4
Some taggers failed. Rerun with -vdebug for errors.
Checking cache...
- app: Not found. Building
- base: Not found. Building
Starting build...
Building [base]...
Sending build context to Docker daemon 3.072kB
...
Successfully built e7294fc7e8ea
Successfully tagged base:f06ab6582db5b2f039bbfe557419b897c31a4161c69c6e7c7832f839b021a7e4
Build [base] succeeded
Building [app]...
Sending build context to Docker daemon 4.608kB
...
Successfully built a8ac08d9dedf
Successfully tagged app:latest
Build [app] succeeded
```

## Steps to reproduce the behavior

1. Go to examples/simple-artifact-dependency of the skaffold repostory.
2. Change skaffold.yaml to:
```yaml
apiVersion: skaffold/v4beta13
kind: Config
build:
tagPolicy:
inputDigest: {}
artifacts:
- image: app
context: app
requires:
- image: base
alias: BASE
- image: base
context: base
manifests:
rawYaml:
- app/k8s-pod.yaml
```
3. Change app/Dockerfile to
```dockerfile
ARG BASE
FROM $BASE AS base

FROM golang:1.18 AS builder
WORKDIR /code
COPY main.go .
COPY go.mod .
# `skaffold debug` sets SKAFFOLD_GO_GCFLAGS to disable compiler optimizations
ARG SKAFFOLD_GO_GCFLAGS
RUN go build -gcflags="${SKAFFOLD_GO_GCFLAGS}" -trimpath -o /app .

FROM scratch
# Define GOTRACEBACK to mark this container as using the Go language runtime
# for `skaffold debug` (https://skaffold.dev/docs/workflows/debug/).
ENV GOTRACEBACK=single
CMD ["./app"]
COPY --from=builder /app .
COPY --from=base hello.txt .
```
4. `skaffold build --push=false` or `skaffold build --dry-run`