Fix missing "f" modifier in format statement in run-libcore-tests.py

Bug: 208639267
Test: art/tools/run-libcore-tests.sh --mode=device libcore.java.text
Change-Id: Ia341e4c1c26e1ee2e7ecce17736ba0819e8429f9
diff --git a/tools/run-libcore-tests.py b/tools/run-libcore-tests.py
index 5c94989..394a4e4 100755
--- a/tools/run-libcore-tests.py
+++ b/tools/run-libcore-tests.py
@@ -418,7 +418,7 @@
     if ART_TEST_CHROOT:
       cmd.append(f"--chroot {ART_TEST_CHROOT} --device-dir=/tmp/vogar/test-{test_name}")
     else:
-      cmd.append("--device-dir=/data/local/tmp/vogar/test-{test_name}")
+      cmd.append(f"--device-dir=/data/local/tmp/vogar/test-{test_name}")
     cmd.append(f"--vm-command={ART_TEST_ANDROID_ROOT}/bin/art")
   else:
     cmd.append(f"--device-dir=/tmp/vogar/test-{test_name}")