Update sizbot to pull artifacts from public mirror

We can't use the normal download-build script in sizebot because it
depends on the CircleCI artifacts API, which was recently changed to
require authorization. And we can't pass an authorization token
without possibly leaking it to the public, since we run sizebot on
PRs from external contributors. As a temporary workaround, this job
will pull the artifacts from a public mirror that I set up. But we
should find some other solution so we don't have to maintain
the mirror.

.circleci/config.yml

@@ -126,23 +126,19 @@ jobs:
     docker: *docker
     environment: *environment
     steps:
-      - checkout
-      - run: yarn workspaces info | head -n -1 > workspace_info.txt
-      - *restore_node_modules
       - run:
           name: Download artifacts for base revision
+          # TODO: We can't use the normal download-build script here because it
+          # depends on the CircleCI artifacts API, which was recently changed to
+          # require authorization. And we can't pass an authorization token
+          # without possibly leaking it to the public, since we run sizebot on
+          # PRs from external contributors. As a temporary workaround, this job
+          # will pull the artifacts from a public mirror that I set up. But we
+          # should find some other solution so we don't have to maintain
+          # the mirror.
           command: |
-              git fetch origin main
-              cd ./scripts/release && yarn && cd ../../
-              scripts/release/download-experimental-build.js --commit=$(git merge-base HEAD origin/main)
-              mv ./build ./base-build
-      - run:
-          # TODO: The `download-experimental-build` script copies the npm
-          # packages into the `node_modules` directory. This is a historical
-          # quirk of how the release script works. Let's pretend they
-          # don't exist.
-          name: Delete extraneous files
-          command: rm -rf ./base-build/node_modules
+            curl -L --retry 60 --retry-delay 10 --retry-max-time 600 https://react-builds.vercel.app/api/commits/$(git merge-base HEAD origin/main)/artifacts/build.tgz | tar -xz
+            mv ./build ./base-build
 
       - persist_to_workspace:
           root: .