Make Windows builds always portable
Make Windows builds always portable

Apparently there is little or no need for a non-portable Windows build, so
rather than trying to figure out which version of Windows we are building
on, just ignore -PortableBuild=false.  We can add a warning or refuse to
accept the switch later if necessary, but for now we need to continue
accepting it to avoid build breaks.

Fixes #14291

cshung/coreclr,mmitche/coreclr,krk/coreclr,cshung/coreclr,poizan42/coreclr,poizan42/coreclr,mmitche/coreclr,mmitche/coreclr,cshung/coreclr,krk/coreclr,poizan42/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,poizan42/coreclr,mmitche/coreclr,krk/coreclr,cshung/coreclr,mmitche/coreclr,poizan42/coreclr,wtgodbe/coreclr,krk/coreclr,wtgodbe/coreclr,mmitche/coreclr,wtgodbe/coreclr,cshung/coreclr,cshung/coreclr,krk/coreclr,poizan42/coreclr,krk/coreclr