Fix sizebot not working due to missing auth token (facebook#26423)

Sizebot works by fetching the base artifacts from CI. CircleCI recently
updated this endpoint to require an auth token. This is a problem for PR
branches, where sizebot runs, because we don't want to leak the token to
arbitrary code written by an outside contributor.

This only affects PR branches. CI workflows that run on the main branch
are allowed to access environment variables, because only those with
push access can land code in main.

As a temporary workaround, we'll fetch the assets from a mirror,
react-builds.vercel.app. This is the same app that hosts the sizebot
diff previews.

Need to figure out a longer term solution. Perhaps by converting sizebot
into a proper GitHub app.

.circleci/config.yml

@@ -151,11 +151,33 @@ jobs:
       - setup_node_modules
       - run:
           name: Download artifacts for base revision
+          # TODO: The download-experimental-build.js script works by fetching
+          # artifacts from CI. CircleCI recently updated this endpoint to
+          # require an auth token. This is a problem for PR branches, where
+          # sizebot needs to run, because we don't want to leak the token to
+          # arbitrary code written by an outside contributor.
+          #
+          # This only affects PR branches. CI workflows that run on the main
+          # branch are allowed to access environment variables, because only those
+          # with push access can land code in main.
+          #
+          # As a temporary workaround, we'll fetch the assets from a mirror.
+          # Need to figure out a longer term solution for this.
+          #
+          # Original code
+          #
+          # command: |
+          #     git fetch origin main
+          #     cd ./scripts/release && yarn && cd ../../
+          #     scripts/release/download-experimental-build.js --commit=$(git merge-base HEAD origin/main) --allowBrokenCI
+          #     mv ./build ./base-build
+          #
+          # Workaround. Fetch the artifacts from react-builds.vercel.app. This
+          # is the same app that hosts the sizebot diff previews.
           command: |
-              git fetch origin main
-              cd ./scripts/release && yarn && cd ../../
-              scripts/release/download-experimental-build.js --commit=$(git merge-base HEAD origin/main) --allowBrokenCI
+            curl -L --retry 60 --retry-delay 10 --retry-max-time 600 https://react-builds.vercel.app/api/commits/$(git merge-base HEAD origin/main)/artifacts/build.tgz | tar -xz
               mv ./build ./base-build
+
       - run:
           # TODO: The `download-experimental-build` script copies the npm
           # packages into the `node_modules` directory. This is a historical