ci: run `make sparse` as part of the GitHub workflow

Occasionally we receive reviews after patches were integrated, where `sparse` (https://sparse.docs.kernel.org/en/latest/ has more information on that project) identified problems such as file-local variables or functions being declared as global. By running `sparse` as part of our Continuous Integration, we can catch such things much earlier. Even better: developers who activated GitHub Actions on their forks can catch such issues before even sending their patches to the Git mailing list. This addresses https://github.com/gitgitgadget/git/issues/345 Note: Not even Ubuntu 20.04 ships with a new enough version of `sparse` to accommodate Git's needs. The symptom looks like this: add-interactive.c:537:51: error: Using plain integer as NULL pointer To counter that, we download and install the custom-built `sparse` package from the Azure Pipeline that we specifically created to address this issue. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>