luci_swarm: set SHELL environment variable

Set to bash by request.

For golang/go#70163

service/luci_swarm/run

@@ -5,4 +5,4 @@
 [ -n ${builder_name} ] || exit 1
 
 chown -R swarming /home/swarming/.swarming
-setuidgid swarming env HOME=/home/swarming /usr/local/bin/bootstrapswarm -hostname ${builder_name}
+setuidgid swarming env HOME=/home/swarming SHELL=/usr/pkg/bin/bash /usr/local/bin/bootstrapswarm -hostname ${builder_name}