Fix a small bug in fetch_kokoro_prebuilts script

Extract the toolchain only when the build is a successful build.

Test: ./fetch-kokoro-prebuilts [build_id] [target]
Bug: b/326833691
Change-Id: I965d470d47cf79465809181f1c6c77b0f806e88d

fetch_kokoro_prebuilts.py

@@ -64,9 +64,9 @@ def fetch_prebuilts(build_id: str, target: str):
         else:
             print("Download successful!")
 
-        # extract the toolchain
-        tar = os.path.abspath(td) + "/" + os.listdir(td)[0]
-        extract_tarball(target, tar)
+            # extract the toolchain
+            tar = os.path.abspath(td) + "/" + os.listdir(td)[0]
+            extract_tarball(target, tar)
 
 
 # Make sure gsutil is installed.