cmd/makemac: add govc env vars to systemd service file

Fixes golang/go#20224

Change-Id: Iae7d70bdf0950541d774d12077552153ce1a15e0
Reviewed-on: https://go-review.googlesource.com/46332
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: jessfraz

cmd/makemac/makemac.service

@@ -6,6 +6,9 @@ After=network.target
 [Service]
 User=gopher
 Environment="PATH=/home/gopher/bin:/usr/local/bin:/usr/bin"
+# The following file decalres these variables: GOVC_URL GOVC_USERNAME GOVC_PASSWORD GOVC_INSECURE
+# A copy is located at http://go/go-builder-macstadiumd-env-file
+EnvironmentFile=/home/gopher/govc_env_file
 ExecStart=/home/gopher/makemac -auto
 
 [Install]