Minor refactoring of TSC decryption [ci skip]

src/game/scripting/tsc/encryption.rs

@@ -1,6 +1,6 @@
 pub fn decrypt_tsc(buf: &mut [u8]) {
     let half = buf.len() / 2;
-    let key = if let Some(0) = buf.get(half) { 0xf9 } else { (-(*buf.get(half).unwrap() as isize)) as u8 };
+    let key = if let Some(0) = buf.get(half) { 0x7 } else { *buf.get(half).unwrap() };
     log::info!("Decrypting TSC using key {:#x}", key);
 
     for (idx, byte) in buf.iter_mut().enumerate() {
@@ -8,6 +8,6 @@ pub fn decrypt_tsc(buf: &mut [u8]) {
             continue;
         }
 
-        *byte = byte.wrapping_add(key);
+        *byte = byte.wrapping_sub(key);
     }
 }