b4: fix abuild config path

Signed-off-by: Caleb Connolly <kc@postmarketos.org>

---
Changes in v3:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v2: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230418-abuild-fix-v2-1-bbeb542f0512@postmarketos.org%3E

Changes in v2:
- make flake8 happy
- remove extraneous return

Link to v1: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230418-abuild-fix-v1-1-0f7287adabf5@postmarketos.org%3E

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
{
  "series": {
    "revision": 3,
    "change-id": "20230418-abuild-fix-4d7a9afedc57",
    "base-branch": "master",
    "prefixes": [],
    "history": {
      "v1": [
        "20230418-abuild-fix-v1-1-0f7287adabf5@postmarketos.org"
      ],
      "v2": [
        "20230418-abuild-fix-v2-1-bbeb542f0512@postmarketos.org"
      ]
    }
  }
}