bisector: Copy to subdir of tempdir

Copy to a subdirectory of the temp directory created for the checkout.
The copy operation expects the destination directory to not exist yet.

Change-Id: Ib980f8789245a6350e82204a35fa0453da0b9b15
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/274454
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/recipes/bisector.py b/recipes/bisector.py
index 0f46b7a..f91d195 100644
--- a/recipes/bisector.py
+++ b/recipes/bisector.py
@@ -179,10 +179,10 @@
                                 remote=remote.url,
                                 branch=remote.branch,
                             ),
-                            root=root,
+                            root=root / 'checkout',
                         )
 
-                        with api.context(cwd=root):
+                        with api.context(cwd=checkout.root):
                             repos[remote] = tuple(
                                 api.git.log(
                                     depth=1000,