Minor bugfix: don't truncate the extension in conda-meta filenames

[maresb]

Description

Conda-lock constructs a fake conda-meta directory containing a .json metadata for each package in a lockfile. A typical .json looks like

{
  "name": "astropy",
  "channel": "conda-forge",
  "url": "https://conda.anaconda.org/conda-forge/linux-64/astropy-5.0-py39hce5d2b2_0.tar.bz2",
  "md5": "5a2d9b5961e84cbbac615c2e3fb7e456",
  "build": "py39hce5d2b2_0",
  "build_number": 0,
  "version": "5.0",
  "subdir": "linux-64",
  "fn": "astropy-5.0-py39hce5d2b2_0.tar.bz2",
  "depends": [
    "importlib-metadata",
    "libgcc-ng >=9.4.0",
    "numpy >=1.19.5,<2.0a0",
    "packaging >=19.0",
    "pyerfa >=2.0",
    "python >=3.9,<3.10.0a0",
    "python_abi 3.9.* *_cp39",
    "pyyaml >=3.13"
  ]
}

Before this change, the fn entry was missing the .tar.bz2 extension. This was leading to problems with micromamba v2 which apparently strengthened the validation logic a bit.

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