add swift 5.4.2

runtimes/swift.nix

@@ -0,0 +1,27 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.swift;
+in piston.mkRuntime {
+    language = "swift";
+    version = "5.4.2";
+    aliases = [];
+
+    run = ''
+        ls ${pkg}
+        ${pkg}/bin/swift -module-cache-path . "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.swift" = ''
+                    print("OK");
+                '';
+            };
+            args = [];
+            stdin = "";
+            packages = [];
+            main = "test.swift";
+        })
+    ];
+}