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GH-34621: [GLib] Don't use "g_strdup(XXX->ToString().c_str())" (#34624)

### Rationale for this change

Because it reports "object backing the pointer will be destroyed at the end of the full-expression" [-Wdangling-gsl] warning.

### What changes are included in this PR?

Create a variable for temporary `std::string`. 

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #34621

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