Support building with python3 on unix (#19356)
Support building with python3 on unix (#19356)

The windows build scripts try finding python in order of python3,
python2 and then python. The unix build scripts dont. They just try
python2 variants and then fail. This change makes brings them closer
together by letting users build using only python3.
wtgodbe/coreclr,wtgodbe/coreclr,cshung/coreclr,mmitche/coreclr,poizan42/coreclr,mmitche/coreclr,krk/coreclr,wtgodbe/coreclr,krk/coreclr,cshung/coreclr,cshung/coreclr,poizan42/coreclr,cshung/coreclr,poizan42/coreclr,mmitche/coreclr,cshung/coreclr,poizan42/coreclr,mmitche/coreclr,cshung/coreclr,poizan42/coreclr,poizan42/coreclr,wtgodbe/coreclr,krk/coreclr,krk/coreclr,mmitche/coreclr,krk/coreclr,mmitche/coreclr,wtgodbe/coreclr,krk/coreclr,wtgodbe/coreclr