[recipe_modules/git] Set checkout_dir in tests/number.py.

It's soon going to be invalid to use checkout_dir before it's set.

R=bryner, recipe-mega-autoroller@chops-service-accounts.iam.gserviceaccount.com

Change-Id: I48185dec02fc9de250bb12843ac9a8bcabbd74d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5342874
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

recipes/README.recipes.md

@@ -1097,10 +1097,10 @@ Raises:
 — **def [RunSteps](/recipes/recipe_modules/git/examples/full.py#20)(api):**
 ### *recipes* / [git:tests/number](/recipes/recipe_modules/git/tests/number.py)
 
-[DEPS](/recipes/recipe_modules/git/tests/number.py#9): [git](#recipe_modules-git), [recipe\_engine/assertions][recipe_engine/recipe_modules/assertions], [recipe\_engine/properties][recipe_engine/recipe_modules/properties]
+[DEPS](/recipes/recipe_modules/git/tests/number.py#9): [git](#recipe_modules-git), [recipe\_engine/assertions][recipe_engine/recipe_modules/assertions], [recipe\_engine/path][recipe_engine/recipe_modules/path], [recipe\_engine/properties][recipe_engine/recipe_modules/properties]
 
 
-— **def [RunSteps](/recipes/recipe_modules/git/tests/number.py#16)(api):**
+— **def [RunSteps](/recipes/recipe_modules/git/tests/number.py#17)(api):**
 ### *recipes* / [git\_cl:examples/full](/recipes/recipe_modules/git_cl/examples/full.py)
 
 [DEPS](/recipes/recipe_modules/git_cl/examples/full.py#6): [git\_cl](#recipe_modules-git_cl), [recipe\_engine/path][recipe_engine/recipe_modules/path], [recipe\_engine/raw\_io][recipe_engine/recipe_modules/raw_io], [recipe\_engine/step][recipe_engine/recipe_modules/step]

recipes/recipe_modules/git/tests/number.py

@@ -9,11 +9,15 @@ PYTHON_VERSION_COMPATIBILITY = 'PY3'
 DEPS = [
     'git',
     'recipe_engine/assertions',
+    'recipe_engine/path',
     'recipe_engine/properties',
 ]
 
 
 def RunSteps(api):
+  # Set the checkout_dir because the `git` module implicitly uses this.
+  api.path.checkout_dir = api.path['cache'].join('builder')
+
   numbers = api.git.number(
       commitrefs=api.properties.get('commitrefs'),
       test_values=api.properties.get('test_values'),