rapids-cmake allow GENERATE_PINNED_VERSIONS via CMake variable

[robertmaynard]

Description

Fixes #552

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[robertmaynard]

Blocked by https://github.com/rapidsai/rapids-cmake/pull/601 that refactors the cpm pinned file testing infrastructure.

[robertmaynard]

/merge