Use Unix Shell build-in 'hash' over 'which'

`which` is an external utility. `hash` or `command -v` are built-in
General Commands.

See http://stackoverflow.com/a/677212/863980

Related: dotnet/coreclr#8823 & dotnet/corert#2831

I chose `hash` here as `run-build.sh` is already using it.

scripts/obtain/dotnet-install.sh

@@ -122,8 +122,8 @@ get_current_os_name() {
 
 machine_has() {
     eval $invocation
-    
-    which "$1" > /dev/null 2>&1
+
+    hash "$1" > /dev/null 2>&1
     return $?
 }