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GH-44234: [CI][C++][AppVeyor] Use conda instead of Mamba (#44235)

### Rationale for this change

`mamba update` with Mamba 2.0.0 failed.

### What changes are included in this PR?

Use conda instead of Mamba. "conda-libmamba-solver" isn't used too. "classic" solver is used.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #44234

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>