bootstrap_python3: Find 'cipd' in base_dir, not PATH

There might be multiple copies of depot_tools involved, and it might not
even be on PATH.

This matches the pattern of various other setup/bootstrap scripts (e.g.,
cipd_bin_setup.sh).

BUG=none
TEST=`.../ensure_bootstrap` without being on PATH

Change-Id: Ic9135298629726e69619d259af110684d017d0f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4377789
Commit-Queue: Brian Norris <briannorris@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>

bootstrap_python3

@@ -25,7 +25,7 @@ function bootstrap_python3 {
   BOOTSTRAP_PATH="bootstrap-${PYTHON3_VERSION}_bin"
 
   # Install CIPD packages. The CIPD client self-bootstraps.
-  "cipd" ensure -log-level warning -ensure-file "${CIPD_MANIFEST}" \
+  "$base_dir/cipd" ensure -log-level warning -ensure-file "${CIPD_MANIFEST}" \
       -root "$BOOTSTRAP_PATH"
 
   BOOTSTRAP_PYTHON_BIN="${BOOTSTRAP_PATH}/python3/bin/python3"