add a comment explaining the setting of $HOME in TestHttpsInsecure (#306)

internal/driver/fetch_test.go

 		t.Fatal("creating temp dir: ", err)
 	}
 	defer os.RemoveAll(tempdir)
+
+	// pprof writes to $HOME/pprof by default which is not necessarily
+	// writeable (e.g. on a Debian buildd) so set $HOME to something we
+	// know we can write to for the duration of the test.
 	os.Setenv(homeEnv(), tempdir)
 	defer os.Setenv(homeEnv(), saveHome)