We want to increase testing timeouts for GitLab CI, where we use our own
runners and observe stalls and high disk pressure when several vinyl
tests are run in parallel. The idea is to set variables in GitLab CI web
interface and read them from test-run (see [1]).

First, we need to pass the variables into inner environments. GitLab CI
jobs run the testing using packpack, Docker or VirtualBox.

Packpack already preserves environment variables that are listed in the
PRESERVE_ENVVARS variable (see [2]).

This commit passes the variables that are listed in the PRESERVE_ENVVARS
variable into Docker and VirtualBox environment. So, all jobs will have
given variables in the enviroment. (Also dropped unused EXTRA_ENV
variable.)

The next commit will update the test-run submodule with support of
setting timeouts using environment variables.

[1]: https://github.com/tarantool/test-run/issues/258
[2]: https://github.com/packpack/packpack/pull/135