kernel-build-trigger should share mirror dir

[khilman]

When the build-trigger runs on a machine with multiple executors, it should share the git mirror.

e.g. the first job runs kci_build update_mirror with: --mirror=/work/kernel/jenkins/workspace/kernel-build-trigger/linux.git

and the second one uses: --mirror=/work/kernel/jenkins/workspace/kernel-build-trigger@2/linux.git
Which means we have multiple copies of the git mirror on the same host.