Unpin MacOS CI builds from CMake 3.19.3

The hosted GitHub Actions runners for macOS recently upgraded the default CMake version to v3.19.1, leading to build failures: https://github.com/aws/aws-encryption-sdk-c/issues/650

We implemented a workaround that involves downgrading CMake to the prior version, v3.18.5: https://github.com/aws/aws-encryption-sdk-c/pull/655

We should root cause the build failure, and determine (a) what changes to make to our build configuration so that we can use newer CMake versions, or (b) whether we need to just wait for a CMake version with the regression patched. Afterwards we should revert the workaround.

Start with issue #650 and pull request #655, then reproduce the macOS GitHub Actions build using the newer CMake versions mentioned here. Done means identifying the regression or required build-configuration change, confirming the build works with a newer version, and reverting the downgrade workaround.

Written by the indexing model from the issue text.