Implement SSH key file path env substitution

Supports all platforms, including Windows with %FOO% syntax.

Fixes #3523

tests/TestTools.h

@@ -27,6 +27,7 @@ private slots:
     void testHumanReadableFileSize();
     void testIsHex();
     void testIsBase64();
+    void testEnvSubstitute();
 };
 
 #endif // KEEPASSX_TESTTOOLS_H