Add pin for libarchive

[lbdreyer]

The refresh lockfile GHA started failing on Iris this past weekend. The change is that we started getting libarchive from conda-forge rather than main where we previously were getting it.

The problem with the conda-forge build is that they use the wrong versioning scheme for their SOs, and this doesn't seem compatible with mamba.

We get the following error with mamba:

" ImportError: libarchive.so.18: cannot open shared object file: No such file or directory\n\nTry (re)installing conda-libmamba-solver."

so it's looking for libarchive.so.18

This is available in the main build:

$ ls -1 $CONDA_PREFIX/lib/libarchive*
/path/to/env/lib/libarchive.a
/path/to/env/lib/libarchive.so
/path/to/env/lib/libarchive.so.18

but not in the conda-forge build:

$ ls -1 $CONDA_PREFIX/lib/libarchive*
/path/to/env/lib/libarchive.a
/path/to/env/lib/libarchive.so
/path/to/env/lib/libarchive.so.13
/path/to/env/lib/libarchive.so.13.5.2

Further details are on this issue: conda-forge/libarchive-feedstock#69

So for now we need to a specific version of libarchive.

[bjlittle]

@lbdreyer Blimey.

Okay, let's roll with the pin for now.

I'll create an issue to make sure that we unpin and reference this PR for context 👍