validate fix: `dotnetup install` doesn't work with an `--install-path` set to a relative directory

### Describe the bug

I'm working on samples using dotnetup, but when I use a [relative path for `--install-root`](https://github.com/baronfel/dotnetup-repo-patterns-demo/actions/runs/23301224664/workflow#L40) the command [doesn't actually seem to create the destination directory](https://github.com/baronfel/dotnetup-repo-patterns-demo/actions/runs/23301224664/job/67762578604#step:8:13).

When I move to a [fully-qualified path](https://github.com/baronfel/dotnetup-repo-patterns-demo/actions/runs/23301819800/workflow#L43) the [directory contents appear correct](https://github.com/baronfel/dotnetup-repo-patterns-demo/actions/runs/23301819800/job/67764749151#step:7:1).