Bug? "More than one build script specified" when calling build.ps1 with a custom cake script and extra parameters

### What You Are Seeing?

When I specify a custom cake script with extra parameters using the build.ps1 bootstrap file, I get the following error:

More than one build script specified.

Note: If I remove the extra parameter that I would like to pass to my script everything works
### What is Expected?

Cake executes using the custom cake script file
### What version of Cake are you using?

0.13.0
### Are you running on a 32 or 64 bit system?

64
### What environment are you running on? Windows? Linux? Mac?

Windows
### Are you running on a CI Server? If so, which one?

None
### How Did You Get This To Happen? (Steps to Reproduce)

.\build.ps1 -Script "3rdPartyPackages.cake" -Settings=3rdPartyPackages.settings.json
### Output Log

```
.\build.ps1 -Script "3rdPartyPackages.cake" -Settings="3rdPartyPackages.settings.json"
Preparing to run build script (3rdPartyPackages.cake)...
Running build script...
& "E:\Projects\tools\Cake\Cake.exe" "3rdPartyPackages.cake" -target="Default" -configuration="Release" -verbosity="Verbose" -Settings=3rdPartyPackages.settin
gs\.json
More than one build script specified.

Usage: Cake.exe [build-script] [-verbosity=value]
[-showdescription] [-dryrun] [..]
```

Start with build.ps1 and the Cake.exe command shown in the output, comparing how the custom script and extra Settings parameter are forwarded. Reproduce the command on the reported Windows setup, then verify that the custom script runs with the extra parameter without the “More than one build script specified” error.

Written by the indexing model from the issue text.