Rollup merge of #84177 - ehuss:join_paths-err, r=kennytm
Rollup merge of #84177 - ehuss:join_paths-err, r=kennytm

Fix join_paths error display.

On unix, the error from `join_paths` looked like this:

```
path segment contains separator `58`
```

This PR changes it to look like this:

```
path segment contains separator `:`
```

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust