CI: avoid race condition in parallel sphinx-build

4 jobs for master
latest
Status Job ID Name Coverage
  Test
passed #4719
amd64
job_clang_amd64

03:54:07

passed #4718
amd64
job_gcc_amd64

00:51:59

failed #4706
amd64
job_clang_amd64

failed #4705
amd64
job_gcc_amd64