GH-48885: [C++] Add missing curl dependency of Arrow::arrow_static CMake target

[kou]

Rationale for this change

We changed macro(build_google_cloud_cpp_storage) to function(...) in GH-48333 . So find_curl() doesn't change ARROW_SYSTEM_DEPENDENCIES in parent scope. (function() creates a new scope.)

What changes are included in this PR?

Move find_curl() to the top-level from in function(build_google_cloud_cpp_storage).

Are these changes tested?

Yes.

Are there any user-facing changes?

Yes.

• GitHub Issue: [C++][CMake] Export config for static libarrow build broken in 23 #48885

[github-actions]

⚠️ GitHub issue #48885 has been automatically assigned in GitHub to PR creator.

[raulcd]

CI failures are unrelated and should be fixed by:

• #48894
• #48890

Let's wait and rebase to have a clean CI.

[kou]

Rebased. This is ready. CI failures are fixed.

[raulcd]

LGTM.
@kou if I understand correctly the changes on the verify-xxx.sh scripts are so this will be surfaced on the Linux packaging jobs so we can prevent for these type of things happening again before releasing?

[kou]

Correct. We can detect similar problems by new checks in verify-XXX.sh.