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GH-34296: [C++][CI] Force appveyor builds to use conda-forge and ignore defaults channel (#34297)

For some reason (potentially related to https://github.com/mamba-org/mamba/issues/656 or maybe just the default conda behavior) it appears that specifying `-c conda-forge` is not sufficient as conda will still check the defaults channel.

Lately it seems the defaults channel has introduced some incompatibility that leads to a deadlock or hang during solve.

This PR disables defaults completely which seems to avoid the hang.
* Closes: #34296

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>