Fix TypeError when conda 26.x returns dict from fetch_latest()

[gabrielbusta]

Description

Hi I ran into an issue running an installer build, and I think it is a constructor/conda compatibility issue:

2026-04-03T16:14:40.0074762Z Traceback (most recent call last):
2026-04-03T16:14:40.0116947Z   File "D:\a\_temp\_constructor_workdir\constructor\Scripts\constructor-script.py", line 10, in <module>
2026-04-03T16:14:40.0118364Z     sys.exit(main())
2026-04-03T16:14:40.0119033Z              ~~~~^^
2026-04-03T16:14:40.0121096Z   File "D:\a\_temp\_constructor_workdir\constructor\Lib\site-packages\constructor\main.py", line 610, in main
2026-04-03T16:14:40.0122090Z     main_build(
2026-04-03T16:14:40.0122516Z     ~~~~~~~~~~^
2026-04-03T16:14:40.0122951Z         dir_path,
2026-04-03T16:14:40.0123651Z         ^^^^^^^^^
2026-04-03T16:14:40.0124099Z     ...<7 lines>...
2026-04-03T16:14:40.0124542Z         debug=args.debug,
2026-04-03T16:14:40.0125031Z         ^^^^^^^^^^^^^^^^^
2026-04-03T16:14:40.0125483Z     )
2026-04-03T16:14:40.0125883Z     ^
2026-04-03T16:14:40.0127095Z   File "D:\a\_temp\_constructor_workdir\constructor\Lib\site-packages\constructor\main.py", line 404, in main_build
2026-04-03T16:14:40.0128009Z     create(info, verbose=verbose)
2026-04-03T16:14:40.0128390Z     ~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
2026-04-03T16:14:40.0129094Z   File "D:\a\_temp\_constructor_workdir\constructor\Lib\site-packages\constructor\winexe.py", line 371, in create
2026-04-03T16:14:40.0129611Z     preconda_write_files(info, tmp_dir)
2026-04-03T16:14:40.0129868Z     ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
2026-04-03T16:14:40.0130367Z   File "D:\a\_temp\_constructor_workdir\constructor\Lib\site-packages\constructor\preconda.py", line 192, in write_files
2026-04-03T16:14:40.0130909Z     write_index_cache(info, pkgs_dir, all_dists)
2026-04-03T16:14:40.0131193Z     ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
2026-04-03T16:14:40.0131723Z   File "D:\a\_temp\_constructor_workdir\constructor\Lib\site-packages\constructor\preconda.py", line 81, in write_index_cache
2026-04-03T16:14:40.0133286Z     repodatas = {url: get_repodata(url) for url in _urls if url is not None}
2026-04-03T16:14:40.0133648Z                       ~~~~~~~~~~~~^^^^^
2026-04-03T16:14:40.0134180Z   File "D:\a\_temp\_constructor_workdir\constructor\Lib\site-packages\constructor\conda_interface.py", line 134, in get_repodata
2026-04-03T16:14:40.0134770Z     full_repodata = json.loads(raw_repodata_str)
2026-04-03T16:14:40.0135203Z   File "D:\a\_temp\_constructor_workdir\constructor\Lib\json\__init__.py", line 345, in loads
2026-04-03T16:14:40.0135985Z     raise TypeError(f'the JSON object must be str, bytes or bytearray, '
2026-04-03T16:14:40.0136636Z                     f'not {s.__class__.__name__}')
2026-04-03T16:14:40.0137227Z TypeError: the JSON object must be str, bytes or bytearray, not dict

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[conda-bot]

We require contributors to sign our Contributor License Agreement and we don't have one on file for @gabrielbusta.

In order for us to review and merge your code, please e-sign the Contributor License Agreement PDF. We then need to manually verify your signature, merge the PR (conda/infrastructure#1308), and ping the bot to refresh the PR.

[marcoesters]

Thank you! This looks good, but we need a news file to this PR as described in the checklist. Please also let me know CLA has been signed and I can get the bot CLA check to re-run.