ci: fix possible dependency issues

Dependencies might break if there is a mirror update between
`pacman -Syu` and `pacman -Sy clang18`.
So do not update the database before installing clang18.

See also the warning in https://wiki.archlinux.org/title/Pacman#Installing_packages

utils/tests/gitlab-ci-doc.yml

@@ -15,7 +15,7 @@
     # temporary revert to Doxygen 1.11.0. The package has been recompiled
     # without spdlog and wizard support to fix a library dependency.
     # Clang-18 is needed by doxygen
-    - pacman -Sy --noconfirm clang18
+    - pacman -S --noconfirm clang18
     - wget -nv http://www.nsnam.org/packages/doxygen-1.11.0-4-x86_64.pkg.tar.zst
     - pacman --noconfirm -U doxygen-1.11.0-4-x86_64.pkg.tar.zst
     # no policy is a good policy.