don't attempt to access linux keychain on non-linux systems, even if classes are present (i.e. when starting from IDE)

[ci skip]

main/keychain/src/main/java/org/cryptomator/keychain/LinuxKeychainTester.java

@@ -1,12 +1,15 @@
 package org.cryptomator.keychain;
 
+import org.apache.commons.lang3.SystemUtils;
+
 import java.util.Optional;
 
 public class LinuxKeychainTester {
+
 	public static boolean secretServiceIsAvailable() {
 		try {
 			Class.forName("org.freedesktop.secret.simple.SimpleCollection");
-			return true;
+			return SystemUtils.IS_OS_LINUX; // even if the classes could be loaded, secretService is only available on linux
 		} catch (ClassNotFoundException e) {
 			return false;
 		}