[ci] Combine sizebot jobs (facebook#32333)

There's no real reason to have 2 jobs for sizebot. It's more of a
historical artifact from before the GH migration. Merging them should
require one less worker needing to be provisioned and some of the extra
overhead
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/32333).
* __->__ facebook#32333
* facebook#32332

Author: poteto

.github/workflows/runtime_build_and_test.yml

@@ -585,9 +585,10 @@ jobs:
           RELEASE_CHANNEL: experimental
 
   # ----- SIZEBOT -----
-  download_base_build_for_sizebot:
-    if: ${{ github.event_name == 'pull_request' && github.ref_name != 'main' }}
-    name: Download base build for sizebot
+  sizebot:
+    if: ${{ github.event_name == 'pull_request' && github.ref_name != 'main' && github.event.pull_request.base.ref == 'main' }}
+    name: Run sizebot
+    needs: [build_and_lint]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -603,50 +604,23 @@ jobs:
         id: node_modules
         with:
           path: "**/node_modules"
-          key: ${{ runner.arch }}-${{ runner.os }}-modules-${{ hashFiles('yarn.lock', 'scripts/release/yarn.lock') }}
+          key: runtime-node_modules-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
       - name: Ensure clean build directory
         run: rm -rf build
-      - run: yarn install --frozen-lockfile
       - run: yarn install --frozen-lockfile
         working-directory: scripts/release
       - name: Download artifacts for base revision
         run: |
-          git fetch origin main
-          GH_TOKEN=${{ github.token }} scripts/release/download-experimental-build.js --commit=$(git rev-parse origin/main)
+          GH_TOKEN=${{ github.token }} scripts/release/download-experimental-build.js --commit=$(git rev-parse ${{ github.event.pull_request.base.sha }})
           mv ./build ./base-build
       # 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
         run: rm -rf ./base-build/node_modules
-      - name: Display structure of base-build
+      - name: Display structure of base-build from origin/main
         run: ls -R base-build
-      - name: Archive base-build
-        uses: actions/upload-artifact@v4
-        with:
-          name: base-build
-          path: base-build
-
-  sizebot:
-    name: Run sizebot
-    needs: [build_and_lint, download_base_build_for_sizebot]
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-        with:
-          ref: ${{ github.event.pull_request.head.sha || github.sha }}
-      - uses: actions/setup-node@v4
-        with:
-          node-version-file: '.nvmrc'
-          cache: yarn
-          cache-dependency-path: yarn.lock
-      - name: Restore cached node_modules
-        uses: actions/cache@v4
-        id: node_modules
-        with:
-          path: "**/node_modules"
-          key: runtime-node_modules-${{ runner.arch }}-${{ runner.os }}-${{ hashFiles('yarn.lock') }}
       - name: Ensure clean build directory
         run: rm -rf build
       - run: yarn install --frozen-lockfile
@@ -662,13 +636,6 @@ jobs:
           node ./scripts/print-warnings/print-warnings.js > build/__test_utils__/ReactAllWarnings.js
       - name: Display structure of build for PR
         run: ls -R build
-      - name: Restore archived base-build from origin/main
-        uses: actions/download-artifact@v4
-        with:
-          name: base-build
-          path: base-build
-      - name: Display structure of base-build from origin/main
-        run: ls -R base-build
       - run: echo ${{ github.sha }} >> build/COMMIT_SHA
       - run: node ./scripts/tasks/danger
       - name: Archive sizebot results