Add a .gitlab-ci.yml variable to run generated [ba]sh scripts under `-x` or with `set -o xtrace`

With issues like https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1705 , debugging would be much easier if each command in the shell script were output to STDOUT before being run. This is the '-x' argument to sh, and leads to output like this:

#!/bin/sh -x
echo ok

+echo ok
ok

@tmaczukin suggests it's most sensible to enable this with a variable or other setting in the .gitlab-ci.yml file.

Don't know offhand if there is a similar option for cmd.exe or powershell.exe