Fix handling of `-priority=N` argument in build.cmd
Fix handling of `-priority=N` argument in build.cmd

This argument is intended to be passed through directly to msbuild,
but CMD doesn't handle `=` well, so using `-priority=N` with
`build all` didn't work, for instance.

Add special handling for the `-priority` argument to get around
this problem.

yizhang82/coreclr,cshung/coreclr,ruben-ayrapetyan/coreclr,poizan42/coreclr,JosephTremoulet/coreclr,cshung/coreclr,cshung/coreclr,ruben-ayrapetyan/coreclr,JosephTremoulet/coreclr,mmitche/coreclr,yizhang82/coreclr,cshung/coreclr,mmitche/coreclr,ruben-ayrapetyan/coreclr,wtgodbe/coreclr,poizan42/coreclr,krk/coreclr,wtgodbe/coreclr,JosephTremoulet/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,krk/coreclr,poizan42/coreclr,JosephTremoulet/coreclr,yizhang82/coreclr,mmitche/coreclr,yizhang82/coreclr,ruben-ayrapetyan/coreclr,wtgodbe/coreclr,cshung/coreclr,mmitche/coreclr,krk/coreclr,mmitche/coreclr,poizan42/coreclr,poizan42/coreclr,JosephTremoulet/coreclr,mmitche/coreclr,poizan42/coreclr,ruben-ayrapetyan/coreclr,JosephTremoulet/coreclr,cshung/coreclr,krk/coreclr,wtgodbe/coreclr,krk/coreclr,yizhang82/coreclr,krk/coreclr,yizhang82/coreclr,ruben-ayrapetyan/coreclr