MINOR: [CI] Install libncurses-dev from unstable on Debian experimental

[tadeja]

Rationale for this change

Nightly job test-debian-experimental-cpp-gcc-15 fails with this new error:

  #9 60.12 E: Unable to satisfy dependencies. Reached two conflicting assignments:
  #9 60.12    1. libncurses-dev:amd64=6.6+20260608-1 is selected for install because:
  #9 60.12       1. llvm-20-dev:amd64=1:20.1.8-1+b4 is selected for install
  ...

Unstable and experimental both had ncurses 6.6+20260608-1 but with 2nd July addition of ...-2 only to unstable causes temp dependency version skew (libtinfo6).

What changes are included in this PR?

Install libncurses-dev from the default (unstable) before the -t experimental LLVM install

Are these changes tested?

To be run by CI job.

Are there any user-facing changes?

No.

[tadeja]

@github-actions crossbow submit test-debian-experimental-cpp-gcc-15

[github-actions]

Revision: dad110d

Submitted crossbow builds: ursacomputing/crossbow @ actions-096b6859b7

Task	Status
test-debian-experimental-cpp-gcc-15