Fix Partition split_path calculation

Now that we use workdir() in make_image(), we have to change the
Partition initializer to remove the path added by workdir() again
to get the proper path on the host.

Fixes #3242.

mkosi/partition.py

@@ -23,7 +23,10 @@ def from_dict(cls, dict: Mapping[str, Any]) -> "Partition":
             type=dict["type"],
             uuid=dict["uuid"],
             partno=int(partno) if (partno := dict.get("partno")) else None,
-            split_path=Path(p) if ((p := dict.get("split_path")) and p != "-") else None,
+            # We have to translate the sandbox path to the path on the host by removing the /work prefix.
+            split_path=(
+                Path(p.removeprefix("/work")) if ((p := dict.get("split_path")) and p != "-") else None
+            ),
             roothash=dict.get("roothash"),
         )