CXX-2793 Add scan-build tasks to EVG config

[eramongodb]

Resolves CXX-2793. Verified by this patch.

This PR adds five new scan-build tasks to the EVG config. The scan-build tasks mostly mirror those for the C Driver, but are parameterized on polyfill library selection and the C++ standard: three for each C++11 polyfill library configuration, one for C++14 with the bsoncxx implementation, and one for C++17 with the standard library. The mnmlstc/core and Boost tasks are expected to be removed as part of CXX-2797.

The release instructions are updated with a new entry instructing the maintainer to check Coverity for new, unexpected, or high severity issues. This step, as well as the regular preceeding "ensure tests are passing" check now that scan-build tasks are included, should be enough to satisfy the requirements for CXX-2793.

As a drive-by improvement, some EVG post commands were tweaked to avoid noisy error messages (empty tar archive, cd failure, etc.).