Fix issue with missing space in AdditionalArguments

[jwittner]

Ran into this issue below (observe the missing space between "ArgValue" and "-projectPath"):

sue -Project C:\Some\Unity\Project\ -AdditionalArguments "-ArgName ArgValue" -WhatIf
What if: Performing the operation "System.Diagnostics.Process.Start()" on target "C:\Program Files\Unity\Hub\Editor\2018.4.14f1\Editor\Unity.exe -ArgName ArgValue-projectPath "C:\Some\Unity\Project\"".

This was happening to do type inference not resulting in the desired outcomes when building up the arguments. Namely $unityArgs was being converted to a string upon construction rather than on assignment to process.StartInfo.Arguments. New explicit types resolves this issue.