Add and use a function "is_signable_build" when determining to codesign.

I accidentally broke this logic when fixing the code that would skip
codesigning when a commit was not the TOT commit on a release branch; it
turns out that TOT commit was doing extra work - it would never be true
on a presubmit (try) bot.

Added tests the best I could I guess.

Change-Id: I5c11e78266bc957e1bcae8a43226a867207c244a
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/67780
Commit-Queue: Matan Lurey <matanl@google.com>
Reviewed-by: John McDole <codefu@google.com>
Reviewed-by: Chris Bracken <cbracken@google.com>
Reviewed-by: Victoria Ashworth <vashworth@google.com>
(cherry picked from commit 0b0cbc3d67b4f94e455b63cc0e581e9a343c20c4)
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/67820