[Bug]: Local `vminit:latest` install doesn't honor APP_ROOT.

[jglogan]

I have done the following

• I have searched the existing issues
• If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

While validating apple/containerization#379 I wanted to run container integration tests using an alternate APP_ROOT. I found that this doesn't work as expected because the locally build vminit:latest was being installed into the standard app root under Application Support regardless of the value of APP_ROOT.

With local modifications in path/to/containerization:

swift package edit containerization --path path/to/containerization
container system property set image.init vminit:latest
make APP_ROOT=test-data all test integration

Current behavior

• make all installs locally built vminit image under ~/Library/Application Support/com.apple.containerization.
• make integration tests fail due missing vminitd.

Expected behavior

• make all installs locally built vminit image under test-data.
• make integration tests pass.

Environment

- OS: 26
- Xcode: 26
- Container: main + local changes for swift-grpc-2

Relevant log output

N/A

Code of Conduct

• I agree to follow this project's Code of Conduct