fix: allow absolute paths for differential packages (#2397)

## Description

allow absolute paths for differential packages

fixes: #2396

## Type of change

- [X] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Other (security config, docs update, etc)

## Checklist before merging

- [ ] Test, docs, adr added or updated as needed
- [ ] [Contributor Guide
Steps](https://github.com/defenseunicorns/zarf/blob/main/CONTRIBUTING.md#developer-workflow)
followed

src/pkg/packager/creator/normal.go

@@ -50,8 +50,7 @@ type PackageCreator struct {
 
 // NewPackageCreator returns a new PackageCreator.
 func NewPackageCreator(createOpts types.ZarfCreateOptions, cfg *types.PackagerConfig, cwd string) *PackageCreator {
-	// differentials are relative to the current working directory
-	if createOpts.DifferentialPackagePath != "" {
+	if createOpts.DifferentialPackagePath != "" && !filepath.IsAbs(createOpts.DifferentialPackagePath) {
 		createOpts.DifferentialPackagePath = filepath.Join(cwd, createOpts.DifferentialPackagePath)
 	}