Cache the string-escape codec at bot_update start

It looks the following speculative fix for string-escape codec worked
for depot_tools subprocess2: https://crbug.com/912292#c2.

Using the same approach for bot_update.

R=gavinmak@google.com

Bug: 1255228
Change-Id: I3244d34abad23f9fdbf648e24fc8a565df1474cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3210521Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -35,6 +35,12 @@ try:
 except ImportError:  # pragma: no cover
   import urllib.parse as urlparse
 
+# Cache the string-escape codec to ensure subprocess can find it later.
+# See crbug.com/912292#c2 for context.
+# TODO(crbug.com/1227140): Clean up when py2 is no longer supported.
+if sys.version_info.major == 2:
+  codecs.lookup('string-escape')
+
 # How many bytes at a time to read from pipes.
 BUF_SIZE = 256