[#3820] Fix run_slow_tests.sh helper to run correctly with gosu
[#3820] Fix run_slow_tests.sh helper to run correctly with gosu

Moving to the new container switched to using `gosu` and that causes the
`run-as-user.sh` script to not handle environment variables passed as
arguments, correctly. This commit fixes that and restores running the
run_slow_tests.sh script correctly.

akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr