use 'conda' instead of 'mamba' in conda-pack job

[jameslamb]

During the 26.06 release, some workflows have been broken by stale sharded repodata for the rapidsai conda channel.

We put a workaround in place for conda and rattler-build in rapidsai/shared-workflows#564, but that didn't cover mamba.

mamba-based operations are failing on the release job here, and it looks like sharded repodata is being used:

...

Resolving Environment                                                                           ⧖ Starting
Resolving Environment                                                                     ✔ Done (6.9 sec)
warning  libmamba The specification of the environment does not seem solvable in your current setup.
...
error    libmamba Could not solve for environment specs
    The following package could not be installed
    └─ rapids =26.6 * is not installable because it conflicts with any installable versions previously reported.
critical libmamba Could not solve for environment specs

(build link)

Support for suppressing the use of sharded repodata in mamba is very new (mamba-org/mamba#4279), so to avoid needing to rebuild CI images, proposing this PR that just switches mamba uses to conda for now.