Explicitly install 'which' on CentOS

which isn't on CentOS by default and is needed by the dotnet install scripts

build.psm1

@@ -688,7 +688,7 @@ function Start-PSBootstrap {
             sudo apt-get install -y -qq $Deps
         } elseif ($IsCentOS) {
             # Build tools
-            $Deps += "curl", "gcc-c++", "cmake", "make"
+            $Deps += "which", "curl", "gcc-c++", "cmake", "make"
 
             # .NET Core required runtime libraries
             $Deps += "libicu", "libunwind"