ci: Fix the binary size workflow

From the bootstrap error:

     thread 'main' panicked at src/bootstrap/src/core/config/config.rs:3283:21:
    `llvm.download-ci-llvm` cannot be set to `true` on CI. Use `if-unchanged` instead.

So do that.

Author: tgross35

.github/workflows/check-binary-size.yml

@@ -62,10 +62,10 @@ jobs:
           PR_SOURCE_REPO: ${{ github.event.pull_request.head.repo.full_name }}
         run: |
           # Bootstrap config
-          cat <<EOF > config.toml
+          cat <<EOF > bootstrap.toml
           change-id = 9999999
           [llvm]
-          download-ci-llvm = true
+          download-ci-llvm = "if-unchanged"
           [rust]
           incremental = false
           EOF