GIT_SILENT: use qt5.15 iso

2 jobs for master in 14 minutes and 42 seconds (queued for 18 minutes and 47 seconds)
Status Job ID Name Coverage
  Build
passed #81837
build_clazy_clang_tidy

00:08:59

passed #81836
linux kf5-qt5 qt5.15

00:05:43

34.0%