fix swift version

runtimes/swift.nix

@@ -3,7 +3,7 @@ let
     pkg = pkgs.swift;
 in piston.mkRuntime {
     language = "swift";
-    version = "5.4.2"; # pkg.version attribute is missing, so pinning it to 5.4.2
+    version = (builtins.parseDrvName pkg.name).version;
     aliases = [];
 
     run = ''
@@ -23,4 +23,4 @@ in piston.mkRuntime {
             main = "file0.code";
         })
     ];
-}
+}