Fix memory leak in Bun.spawn (#20095)

Co-authored-by: Dylan Conway <dylan.conway567@gmail.com>

src/string/MutableString.zig

@@ -276,7 +276,7 @@ pub fn sliceWithSentinel(self: *MutableString) [:0]u8 {
 }
 
 pub fn toOwnedSliceLength(self: *MutableString, length: usize) string {
-    self.list.shrinkAndFree(self.allocator, length);
+    self.list.items.len = length;
     return self.list.toOwnedSlice(self.allocator) catch bun.outOfMemory(); // TODO
 }