Generally clean up a few things.

.github/actions/build-with-patched-std/action.yml

@@ -21,7 +21,7 @@ outputs:
     description: The size in bytes of the built test binary
     value: ${{ steps.measure.outputs.test-binary-size }}
 runs:
-  using: "composite"
+  using: composite
   steps:
     - shell: bash
       id: measure

.github/actions/report-code-size-changes/action.yml

@@ -30,13 +30,16 @@ runs:
 
           const size_dir = process.env.DATA_DIRECTORY;
 
+          // Map the set of all the *.json files into an array of objects.
           const globber = await glob.create(`${size_dir}/*.json`);
           const files = await globber.glob();
           const sizes = files.map(path => {
             const contents = fs.readFileSync(path);
             return JSON.parse(contents);
           });
 
+          // Map each object into some text, but only if it shows any difference
+          // to report.
           const size_reports = sizes.flatMap(size_data => {
             const platform = size_data["platform"];
             const reference = size_data["reference"];
@@ -91,6 +94,8 @@ runs:
             }
           });
 
+          // If there are any size changes to report, format a comment and post
+          // it.
           if (size_reports.length > 0) {
             const comment_sizes = size_reports.join("");
             const body = `Code size changes for a hello-world Rust program linked with libstd with backtrace:

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

@@ -6,6 +6,7 @@ name: Check binary size
 
 on: pull_request_target
 
+# Both the "measure" and "report" jobs need to know this.
 env:
   SIZE_DATA_DIR: sizes
 
@@ -51,6 +52,8 @@ jobs:
       - name: Set up std repository and backtrace submodule for size test
         shell: bash
         working-directory: ${{ env.RUSTC_DIR }}
+        env:
+          PR_SOURCE_REPO: ${{ github.event.pull_request.head.repo.full_name }}
         run: |
           # Bootstrap config
           cat <<EOF > config.toml
@@ -70,19 +73,19 @@ jobs:
           git submodule update --init library/backtrace
 
           cd library/backtrace
-          git remote add head-pr https://github.com/${{ github.event.pull_request.head.repo.full_name }}
+          git remote add head-pr "https://github.com/$PR_SOURCE_REPO"
           git fetch --all
       - name: Build binary with base version of backtrace
         uses: ./backtrace/.github/actions/build-with-patched-std
         with:
-          backtrace-commit: $BASE_COMMIT
+          backtrace-commit: ${{ env.BASE_COMMIT }}
           main-rs: ${{ env.TEST_MAIN_RS }}
           rustc-dir: ${{ env.RUSTC_DIR }}
         id: size-reference
       - name: Build binary with PR version of backtrace
         uses: ./backtrace/.github/actions/build-with-patched-std
         with:
-          backtrace-commit: $HEAD_COMMIT
+          backtrace-commit: ${{ env.HEAD_COMMIT }}
           main-rs: ${{ env.TEST_MAIN_RS }}
           rustc-dir: ${{ env.RUSTC_DIR }}
         id: size-updated