Fix build.ps1 script failing when spaces exist in source path

[ayan4m1]

Currently, if there are spaces in the path to the CKAN source (e.g. C:\Users\First Last\CKAN), running build.ps1 fails with:

C:\Users\First : The term 'C:\Users\First' is not recognized as the name of a cmdlet, function, script file, or
operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and tr
again.
At line:1 char:1
+ C:\Users\First Last\CKAN/_build/tools/NuGet/4.1.0/nuget.exe instal ...
+ ~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (C:\Users\First:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

Adding an ampersand to the nuget and cake invocations fixes these errors.