if git is not installed skaffold silently falls back to sha256 tagPolicy

When git is not installed skaffold silently falls back to sha256 tagPolicy ignoring gitCommit tagPolicy. I've got hard time debugging why skaffold works as expected locally but wrong in CI, while versions are same

### Expected behavior

FAILURE: git not installed. Can't use specified tagPolicy

### Actual behavior

Silent fallback to sha256 tagPolicy. Images tagged with IMAGE_TAG is set to latest

### Information

- Skaffold version: 1.20.0
- Operating system: all
- Contents of skaffold.yaml:

```yaml
apiVersion: skaffold/v2beta9
kind: Config
metadata:
name: best
build:
artifacts:
- image: registry.dummy/best
context: ./
docker:
dockerfile: Dockerfile
tagPolicy:
gitCommit:
variant: AbbrevCommitSha
local:
push: true
useBuildkit: true
deploy:
helm:
releases:
- name: best
chartPath: helm/best
values:
image: registry.dummy/best
```

### Steps to reproduce the behavior

1. uninstall git
2. `skaffold render`
3. no errors present, image tag set to latest