Fix tail usage on rust-coreutils

Apertis now uses rust-coreutils, which is not fully compatible with
GNU coreutils. Fix the usage of tail according to rust-coreutils
requirements.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

run-test-power-cut.sh

@@ -14,7 +14,7 @@ EOF
 get_phase_data()
 {
 	COMMIT_BEFORE=$(head -1 $(get_phase_data_path))
-	DELAY=$(tail -1 $(get_phase_data_path))
+	DELAY=$(tail -n 1 $(get_phase_data_path))
 }
 
 phase_boot()