[ci/fix] quiet git-clang-format to avoid errors

If git-clang-format is not quiet, it's output changes between "no
modified files to format" and "clang-format did not modify files"
depending on whether the branch contains files to inspect but properly
formatted or no files to inspect at all. This breaks the CI step as is.
6 jobs for clang-format-ci-check in 3 minutes and 19 seconds
Status Job ID Name Coverage
  Style
passed #67353
nix kvm
clang-format

00:00:02

passed #67352
nix kvm
repoquality

00:00:02

passed #67354
nix kvm
style:docs

00:00:27

 
  Build
passed #67355
nix kvm
make:generic

00:00:43

passed #67357
knl
make:knl

00:02:50

passed #67356
nix kvm
make:out-of-tree

00:00:44