Add environment variables to runner

The environment variables are injected to the pods that are created while a build is running.

To verify:

1. Install runner with test values for runners.envVars, e.g. --set runners.env.ENV=value,runners.env.LC_ALL=en_US.UTF-8.
2. Create a test repository, e.g. gitlab-runner-env-vars-test, and use the following for .gitlab-ci.yaml:

check_env:
  script: env | grep 'ENV=value' && env | grep 'LC_ALL=en_US.UTF-8'

Closes gitlab-org/gitlab-runner#3711 (closed)