Using `--registry=file://%workspace%` instead of `--registry=file:///%workspace%` works on Linux but fails on Windows with a cryptic message

### Description of the bug:

When using a local bzlmod registry (using the pretty much undocumented `%workspace%` place holder to use a registry relative to the workspace), I first was using `--registry=file://%workspace%/some/path` which was happily working on Linux, but failing on Windows with

```
ERROR: Error computing the main repository mapping: in module dependency chain -> googletest@1.14.0.bcr.1: Error accessing registry file://%workspace%/some/path: C
```
which is not super helpful. Switching to `--registry=file:///%workspace%/some/path` with a triple `/` has solved the issue, but it wasn't immediate to understand that.

### Which category does this issue belong to?

_No response_

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Create a local bazel registry relative to the workspace, and reference it with `--registry=file://%workspace%` instead of `--registry=///%workspace%`, add at least one `bazel_dep` (doesn't have to be into the local registry, which AFAIK might also be empty), then run on Windows.

### Which operating system are you running Bazel on?

Windows

### What is the output of `bazel info release`?

release 7.1.1

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

_No response_

### Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

I'm supposing that `%workspace%` is being expanded to `C:/...` (or `C:\...`), which makes `file://C:/...` fail just saying `C` because it's an unexpected character? It would probably help having a bit more context to that error. Another possibility is do some explicit checking of `%workspace%` usage in `--registry` values, so that only stuff starting with `file:///%workspace%` is accepted, at least on Windows, and rejected with a helpful error otherwise. Finally, it would also help if `%workspace%` was actually mentioned somewhere in the documentation about `--registry`, maybe with an example usage.