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GH-45270: [C++][CI] Disable mimalloc on Valgrind builds (#45271)

### Rationale for this change

Valgrind is not aware of third-party allocators such as mimalloc and jemalloc. This can lead to spurious errors or, on the contrary, it could hide some actual issues.

### What changes are included in this PR?

Disable mimalloc in the Valgrind build (jemalloc is already disabled).

### Are these changes tested?

Yes, by existing CI.

### Are there any user-facing changes?

No.

* GitHub Issue: #45270

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>