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GH-35662: [CI][C++][MinGW] Avoid crash in FormatTwoDigits() with release build (#35663)

### Rationale for this change

I don't know why but the following combination is crashed:

* Template `FormatTwoDigits()` implementation without explicit `inline`
* MinGW
* Release build

### What changes are included in this PR?

This breaks the "template `FormatTwoDigits()` implementation without explicit `inline`" by specifying `inline` explicitly.

But I don't know why we can avoid this crash by specifying `inline` explicitly...

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #35662

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>