Add dist_dir to goma trigger

This is done so that prototype tooling can write logs inside the DIST_DIR.

Test: I wrote out all the environment variables being passed to
goma_ctl.py and the output is https://paste.googleplex.com/6663469954760704. It contains both DIST_DIR and other variables.

Change-Id: Ia5c70e58aa1b87db9777c59cf3e5df1c9e20f08c