Don't run hooks in a subshell
Don't run hooks in a subshell

Reverts change in 2f064eae4d940877ba04b0a65df4d621315a8568 that
affects modifying the processes environment (such as $PATH) in a hook.

Closes #188

Shopify/agent,buildkite/agent,Shopify/agent,buildkite/agent,Shopify/agent,grosskur/agent,grosskur/agent,buildkite/agent,grosskur/agent