Fix DirectoryToBuild errors w trailing backslash (#34432)
Fix DirectoryToBuild errors w trailing backslash (#34432)

When using the root build script to build a particular project,
the command fails if the supplied project path has a trailiing
backslash and is not the last command.
I.e. `build src\System.Runtime\ /p:p:CoreCLROverridePath=...`
ptoonen/corefx,shimingsg/corefx,wtgodbe/corefx,wtgodbe/corefx,shimingsg/corefx,wtgodbe/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,shimingsg/corefx,ViktorHofer/corefx,ericstj/corefx,shimingsg/corefx,ptoonen/corefx,ViktorHofer/corefx,ViktorHofer/corefx,ViktorHofer/corefx,ptoonen/corefx,BrennanConroy/corefx,ericstj/corefx,ViktorHofer/corefx,ericstj/corefx,ptoonen/corefx,ViktorHofer/corefx,ericstj/corefx,ericstj/corefx,wtgodbe/corefx,ptoonen/corefx,wtgodbe/corefx,ViktorHofer/corefx,wtgodbe/corefx,wtgodbe/corefx,shimingsg/corefx,ptoonen/corefx,ptoonen/corefx,BrennanConroy/corefx,shimingsg/corefx,BrennanConroy/corefx