Bug: Skaffold tagger ignores Docker target when calculating image tags

## Environment
- Skaffold version: v2.16.0
- OS: Linux
- Docker version: Docker version 28.2.2, build e6534b4

## Description
Skaffold generates identical image tags for different build profiles when the only difference is the Docker build target. This causes duplicate images to be built and pushed, wasting CI/CD resources and creating ambiguity about which variant is deployed.

## Steps to Reproduce
1. Create a `Dockerfile` with multiple targets:
```Dockerfile
FROM alpine as base

FROM base as dev
RUN touch devbuild

FROM base as prod
RUN touch prodbuild
```
2. Configure `skaffold.yaml` with different profiles using different targets:
```yaml
apiVersion: skaffold/v4beta11
kind: Config
profiles:
- name: dev
activation:
- command: dev
build:
tagPolicy:
inputDigest: {}
local:
concurrency: 0
useBuildkit: true
tryImportMissing: true
artifacts:
- image: out-image
context: .
docker:
dockerfile: Dockerfile
target: dev
- name: prod
activation:
- command: prod
build:
tagPolicy:
inputDigest: {}
local:
concurrency: 0
useBuildkit: true
tryImportMissing: true
artifacts:
- image: out-image
context: .
docker:
dockerfile: Dockerfile
target: prod
```
3. Run skaffold with different profiles:
```bash
$ skaffold build --profile dev --dry-run
Generating tags...
- out-image -> out-image:dd0bcd52c53d80e2a77a84d416883ce1804609153de66be26341326f945233c5
Skipping build phase since --dry-run=true

$ skaffold build --profile prod --dry-run
Generating tags...
- out-image -> out-image:dd0bcd52c53d80e2a77a84d416883ce1804609153de66be26341326f945233c5
Skipping build phase since --dry-run=true
```
## Expected Behavior
Skaffold should generate different image tags for different profiles when they use different Docker build targets, as they produce functionally different images.

## Actual Behavior
Skaffold generates identical image tags for both profiles:

```
out-image:dd0bcd52c53d80e2a77a84d416883ce1804609153de66be26341326f945233c5
```

## Impact
* Ambiguity about which image variant (dev or production) is actually deployed
* Potential for development images to be deployed in production environments
* CI/CD pipelines build and push redundant images

## Possible Solution
Modify the checksum calculation in the Skaffold tagger to include the Docker target as part of the input when generating image tags.