[C++] cpp/thirdparty/download-dependencies.sh is broken

[asfimport]

This occurred during some of the recent vendoring work

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #5941

_{Note: This issue was originally created as ARROW-7298. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 5941
#5941