use `&` instead of start-process in x.ps1

start-process has weird parsing rules and buggy behavior. we've already had to work around them several times, and the workarounds were not complete.
i wonder who could have added it HMMMMMM
```
PS C:\Users\jyn\src\rust> git log --reverse -S Start-Process x.ps1
commit 775c3c0493e9a383a7f1c521b06d36f2e3d0d886
Author: Jynn Nelson <github@jyn.dev>
Date:   Sun Jul 31 14:02:31 2022 -0500

    Add `x.sh` and `x.ps1` shell scripts
```

the latest broken thing is trailing backslashes:
```
$ x.ps1 t .\tests\ui\error-emitter\
```
would be transformed into
```
['t', '.\\tests\\ui\\error-emitter"']
```

rather than trying to hack around that too, abandon start-process altogether and just use `&`.

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