fix(ci): Fix verification workflow

[paleolimbot]

There are a few problems in the verification workflow:

• On Windows, we weren't exporting any symbols in the C data integration test (when building with CMake)
• The R package testthat doesn't seem to support centos7, which we would need to test the R package there
• The R package jsonlite seems to leak memory. One fix is Free memory allocated by base64_encode() jeroen/jsonlite#442 , but in the meantime we can ignore the leak since it doesn't come from us.