tools: Fix Python exception when uprev tast external file

The regex pattern should be string or it raises TypeError exception.

BUG=b:342335042
TEST=manual tested by uprev new images

Change-Id: Ica38a7327438ab05f57598713f389b7a52744b36
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crospts/+/5579594
Auto-Submit: Darren Wu <darrenwu@chromium.org>
Reviewed-by: David Stevens <stevensd@chromium.org>
Tested-by: Darren Wu <darrenwu@chromium.org>
Commit-Queue: David Stevens <stevensd@chromium.org>