`aspire update` fails to handle Windows-style path separators in project references on Linux/WSL

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

When running `aspire update` on Linux/WSL, the tool fails to update projects if a referenced project uses Windows-style (`\`) path separators in its relative path. The update command cannot open referenced files with backslashes in the path.

### Expected Behavior

`aspire update` should recognize and handle both Windows (`\`) and Unix (`/`) style path separators when resolving project references, regardless of the host OS. It should be able to resolve the referenced .csproj files and update as needed.

### Steps To Reproduce

1. On Linux or WSL, run the following command in a directory containing aspire projects:
```sh
find . -iname '*apphost.csproj' -exec aspire update --project {} --channel staging --non-interactive \;
```
2. Ensure that one of the referenced .csproj files contains Windows-style (`\`) path separators in its ``
3. Observe that `aspire update` fails to update the referenced project, as it cannot open the file with the Windows-style path.

### Exceptions (if any)

_No response_

### .NET Version info

_No response_

### Anything else?

Trying to update aspire samples on WSL (Linux). Encountered this with .csproj files referencing others using backslashes as path separators.