[clang] Remove extra colon from build.py

Follow-up to crrev.com/c/5997052

Change-Id: Ic7ef1c3a53feab8c8a787186c0ffca4fa6dc7fc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6000340
Reviewed-by: Amy Huang <akhuang@google.com>
Cr-Commit-Position: refs/heads/main@{#1379190}
NOKEYCHECK=True
GitOrigin-RevId: cdb9883a5c99bbc77891fedbe0b44a725ed38c81