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GH-45510: [CI][C++] Fix LLVM APT repository preparation on Debian (#45511)

### Rationale for this change

The existing LLVM APT repository preparation is broken. For example, it uses unavailable `${available_llvm}`.

### What changes are included in this PR?

* Use `.asc` for armored key
* Use deb822 format for APT source: https://manpages.debian.org/bookworm/dpkg-dev/deb822.5.en.html

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #45510

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