Add CGO flags for building on MacOS

In the server evergreen.yml files, we historically had CGO_CPPFLAGS=-I/opt/mongodbtoolchain/v2/include CGO_CFLAGS=-mmacosx-version-min=10.11 CGO_LDFLAGS=-mmacosx-version-min=10.11'.

The include path should be unnecessary now that the go-openssl wrapper supports pkg-config, plus we no longer use OpenSSL. However, the minimum version flags might still be relevant to add to Go environment configuration.