[runner] Fix environment specification

"A=B;cmd" doesn't work on fuchsia. Use the session's method of setting
the environment.

Change-Id: Ia30ec0810110bf01147bf775bfdfa97bed95d641
diff --git a/runner/ssh_runner.go b/runner/ssh_runner.go
index 55f4e7a..1917ce5 100644
--- a/runner/ssh_runner.go
+++ b/runner/ssh_runner.go
@@ -23,7 +23,7 @@
 	r.Session.Stderr = stderr
 
 	// TERM-dumb, to avoid a loop fetching a cursor position.
-	command = append([]string{"TERM=dumb;"}, command...)
+	r.Session.Setenv("TERM", "dumb")
 	cmd := strings.Join(command, " ")
 
 	if err := r.Session.Start(cmd); err != nil {