Fix bugs in the Rig build

Signed-off-by: Gavin Howard <gavin@yzena.com>

build.pkg.rig

@@ -175,7 +175,7 @@ fn walk_src_dir() -> []str
 			continue;
 		}
 
-		files +~= f;
+		files! +~= f;
 	}
 
 	dirs: []str = find_dirs "src";
@@ -186,10 +186,10 @@ fn walk_src_dir() -> []str
 
 		foreach f: d_files
 		{
-			files +~= f;
+			files! +~= f;
 		}
 
-		dos = d +~ "/os";
+		dos: str = d +~ "/os";
 
 		if exists dos
 		{
@@ -199,7 +199,7 @@ fn walk_src_dir() -> []str
 
 				foreach f: win_files
 				{
-					files +~= f;
+					files! +~= f;
 				}
 			}
 			else if platform.os == "Mac OSX" && exists (dos +~ "/darwin")
@@ -208,7 +208,7 @@ fn walk_src_dir() -> []str
 
 				foreach f: darwin_files
 				{
-					files +~= f;
+					files! +~= f;
 				}
 			}
 			else
@@ -228,7 +228,7 @@ fn walk_src_dir() -> []str
 						}
 					}
 
-					files +~= f;
+					files! +~= f;
 				}
 			}
 		}