windows: add support for syscall.runtimeSetenv

I missed this in https://github.com/tinygo-org/tinygo/pull/3391 (because
I didn't test on Windows, my fault).

src/runtime/env_windows.go

@@ -0,0 +1,23 @@
+//go:build windows
+
+package runtime
+
+// Set environment variable in Windows:
+//
+//	BOOL SetEnvironmentVariableA(
+//	    [in]           LPCSTR lpName,
+//	    [in, optional] LPCSTR lpValue
+//	);
+//
+//export SetEnvironmentVariableA
+func _SetEnvironmentVariableA(key, val *byte) bool
+
+func setenv(key, val *byte) {
+	// ignore any errors
+	_SetEnvironmentVariableA(key, val)
+}
+
+func unsetenv(key *byte) {
+	// ignore any errors
+	_SetEnvironmentVariableA(key, nil)
+}