Fallback to solved record filename to find the downloaded tarball in get_upstream_pins

[jaimergp]

Description

Closes #4991

After quite some effort debugging this issue, this is what we've found:

• The channel configuration must either redefine default_channels or channel_alias, or both.
• The package cache must be clean (e.g. after conda clean -pt)

In this situation, the get_upstream_pins calculation is forced to download the package on the spot to extract it and retrieve its run_exports metadata.

The problem is caused by a small divergence in the indices (parsed repodatas) as loaded by libmamba and conda, respectively. With customized default_channels and channel_alias values, the channel attribute of each PackageRecord in the conda-build index is set differently than in libmamba.

For example, with this configuration:

channels:
  - defaults
default_channels:
  - conda-forge
solver: libmamba

, the offending block tries to match these two records:

• conda-forge::python-3.11.6-h43d1f9e_0_cpython (provided by libmamba, the solver)
• defaults::python-3.11.6-h43d1f9e_0_cpython (calculated by conda-build's index, which is built separately but not used in practice)

They are the same package with different channel. Since conda-build loads the repodata on its own, it interprets that the conda-forge channel should be called defaults given the config in default_channels. However that's a multichannel and libmamba doesn't handle those well and it prefers to use the actual channel behind the multichannel abstraction (e.g. pkgs/main for the default defaults). Hence the divergence.

I was going to try fix this in conda-libmamba-solver so the indices match, but:

• That's going to be a bigger fix touching an already fragile interface
• The fix in conda-build is insultingly simple. I can't imagine a scenario where the filenames may not match, and I would say the must by design. That means we can reuse the one in the solver-provided PackageRecord. I am only adding it as a fallback if not found in the conda-build index as an abundance of caution.
• This code path will be obsolete soon when we add run_exports.json support (via CEP-11).

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• Add / update outdated documentation?