sandbox: Bind mount on top of symlink if possible

Instead of mounting on top of the resolved symlink, let's just mount
directly on top of the symlink if we can.

mkosi/sandbox.py

@@ -459,11 +459,16 @@ def __eq__(self, other: object) -> bool:
 
     def execute(self, oldroot: str, newroot: str) -> None:
         src = chase(oldroot, self.src)
-        dst = chase(newroot, self.dst)
 
         if not os.path.exists(src) and not self.required:
             return
 
+        # If we're mounting a file on top of a symlink, mount directly on top of the symlink instead of
+        # resolving it.
+        if not os.path.isdir(src) and os.path.islink(self.dst):
+            return mount_rbind(src, self.dst, attrs=MOUNT_ATTR_RDONLY if self.readonly else 0)
+
+        dst = chase(newroot, self.dst)
         if not os.path.exists(dst):
             isfile = os.path.isfile(src)