The `exec` command does not provide a method for specifying a build timeout

The local build command exec is hard coded to a 1800 second timeout and there is no means of customization. The line in question is here:

https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/commands/exec.go#L206

A command line option or an environment variable would be ideal in our environment for setting this parameter greater than 1800 seconds.