[C++] CMAKE_AR is not passed to ZSTD thirdparty dependency

[asfimport]

ZSTD_CMAKE_ARGS should utilize https://github.com/apache/arrow/blob/master/cpp/cmake_modules/ThirdpartyToolchain.cmake#L359

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

PRs and other links:

• GitHub Pull Request #3885

_{Note: This issue was originally created as ARROW-4831. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3885
#3885