gitlab-ci: Switch to clang-tools

clang-3.9 does not contain scan-build anymore.
4 jobs for fedora in 2 minutes and 57 seconds
Status Job ID Name Coverage
  Build
passed #4579
librem5
build-debian-clang

00:02:36

passed #4578
librem5
build-debian-gcc

00:02:11

25.4%
passed #4581
librem5
build-debian-package

00:02:16

passed #4580
librem5
build-fedora-gcc

00:02:56