Fix init-tools.sh for Alpine Linux (#24423)
Fix init-tools.sh for Alpine Linux (#24423)

The URL used to get the dotnet sdk for Alpine was accidentally left
unchanged when the URL for other targets was modified.
I have fixed that and also unified the creation of the dotnet-sdk-XXX
part of the URL for all platforms.
axelheer/corefx,fgreinacher/corefx,Ermiar/corefx,ptoonen/corefx,Ermiar/corefx,parjong/corefx,mmitche/corefx,mmitche/corefx,zhenlan/corefx,axelheer/corefx,ptoonen/corefx,BrennanConroy/corefx,seanshpark/corefx,fgreinacher/corefx,tijoytom/corefx,mmitche/corefx,wtgodbe/corefx,tijoytom/corefx,ravimeda/corefx,Ermiar/corefx,Jiayili1/corefx,fgreinacher/corefx,shimingsg/corefx,twsouthwick/corefx,axelheer/corefx,wtgodbe/corefx,ptoonen/corefx,wtgodbe/corefx,twsouthwick/corefx,parjong/corefx,ravimeda/corefx,Ermiar/corefx,twsouthwick/corefx,Jiayili1/corefx,mmitche/corefx,zhenlan/corefx,ViktorHofer/corefx,Jiayili1/corefx,seanshpark/corefx,shimingsg/corefx,ericstj/corefx,ravimeda/corefx,shimingsg/corefx,ViktorHofer/corefx,tijoytom/corefx,zhenlan/corefx,wtgodbe/corefx,wtgodbe/corefx,zhenlan/corefx,twsouthwick/corefx,Ermiar/corefx,Jiayili1/corefx,ericstj/corefx,shimingsg/corefx,Jiayili1/corefx,ViktorHofer/corefx,parjong/corefx,ericstj/corefx,shimingsg/corefx,twsouthwick/corefx,ravimeda/corefx,tijoytom/corefx,Jiayili1/corefx,ViktorHofer/corefx,mmitche/corefx,ericstj/corefx,zhenlan/corefx,tijoytom/corefx,fgreinacher/corefx,ViktorHofer/corefx,seanshpark/corefx,twsouthwick/corefx,mmitche/corefx,axelheer/corefx,parjong/corefx,mmitche/corefx,tijoytom/corefx,Ermiar/corefx,ViktorHofer/corefx,parjong/corefx,ericstj/corefx,parjong/corefx,twsouthwick/corefx,ravimeda/corefx,BrennanConroy/corefx,seanshpark/corefx,zhenlan/corefx,ptoonen/corefx,Jiayili1/corefx,ericstj/corefx,Ermiar/corefx,ravimeda/corefx,seanshpark/corefx,ptoonen/corefx,wtgodbe/corefx,ViktorHofer/corefx,seanshpark/corefx,zhenlan/corefx,ravimeda/corefx,seanshpark/corefx,axelheer/corefx,ericstj/corefx,wtgodbe/corefx,tijoytom/corefx,shimingsg/corefx,axelheer/corefx,parjong/corefx,ptoonen/corefx,shimingsg/corefx,BrennanConroy/corefx,ptoonen/corefx