Hardcode millBinPlatform to 0.11 (#3353)

Since 0.12.x is meant to be bincompatible with 0.11.x

build.sc

@@ -237,12 +237,13 @@ def millLastTag: T[String] = T {
 }
 
 def millBinPlatform: T[String] = T {
-  val tag = millLastTag()
-  if (tag.contains("-M")) tag
-  else {
-    val pos = if (tag.startsWith("0.")) 2 else 1
-    tag.split("[.]", pos + 1).take(pos).mkString(".")
-  }
+  //val tag = millLastTag()
+  //if (tag.contains("-M")) tag
+  //else {
+  //  val pos = if (tag.startsWith("0.")) 2 else 1
+  //  tag.split("[.]", pos + 1).take(pos).mkString(".")
+  //}
+  "0.11"
 }
 
 def baseDir = build.millSourcePath