CI VMs should not update their locate DB

Found in https://testresults.qt.io/logs/qt/qtbase/a8080e14fc3cbc158e3f31c38ba0b8a8be6e2646/LinuxUbuntu_16_04x86_64LinuxUbuntu_16_04x86_64GCCqtci-linux-Ubuntu-16.04-x86_64-2-3acddcDeveloperBuild_OutOfSourceBuild_QtLibInfix_QtNamespace_BuildExamples_Documentation/2a48a3b7dcb9652b1532a3ad14de7580b5b9b1ee/build_1529947615/log.txt.gz

agent:2018/06/25 17:39:43 build.go:131: |                2625|                        updatedb.mlocate|         D| 13.404504|

Please disable extraneous tasks in the VMs, such as checking for updates, updating the locate database (remove it, since the CI doesn't get lost and needs to ask for help), defragmenting the filesystem, etc.