Install tidy for aarch64-apple-darwin

The GitHub Actions image has this preinstalled for x86_64 but not M1.

.github/workflows/ci.yml

@@ -106,6 +106,9 @@ jobs:
       - name: install clang
         run: src/ci/scripts/install-clang.sh
         if: success() && !env.SKIP_JOB
+      - name: install tidy
+        run: src/ci/scripts/install-tidy.sh
+        if: success() && !env.SKIP_JOB
       - name: install WIX
         run: src/ci/scripts/install-wix.sh
         if: success() && !env.SKIP_JOB
@@ -483,6 +486,9 @@ jobs:
       - name: install clang
         run: src/ci/scripts/install-clang.sh
         if: success() && !env.SKIP_JOB
+      - name: install tidy
+        run: src/ci/scripts/install-tidy.sh
+        if: success() && !env.SKIP_JOB
       - name: install WIX
         run: src/ci/scripts/install-wix.sh
         if: success() && !env.SKIP_JOB
@@ -607,6 +613,9 @@ jobs:
       - name: install clang
         run: src/ci/scripts/install-clang.sh
         if: success() && !env.SKIP_JOB
+      - name: install tidy
+        run: src/ci/scripts/install-tidy.sh
+        if: success() && !env.SKIP_JOB
       - name: install WIX
         run: src/ci/scripts/install-wix.sh
         if: success() && !env.SKIP_JOB

src/ci/github-actions/ci.yml

@@ -173,6 +173,10 @@ x--expand-yaml-anchors--remove:
         run: src/ci/scripts/install-clang.sh
         <<: *step
 
+      - name: install tidy
+        run: src/ci/scripts/install-tidy.sh
+        <<: *step
+
       - name: install WIX
         run: src/ci/scripts/install-wix.sh
         <<: *step

src/ci/scripts/install-tidy.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# This script downloads and installs the tidy binary from Homebrew.
+
+set -euo pipefail
+IFS=$'\n\t'
+
+source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
+
+# Only the macOS arm64/aarch64 GitHub Actions runner needs to have tidy
+# installed; other platforms have it preinstalled.
+
+if isMacOS; then
+    platform=$(uname -m)
+    case $platform in
+        x86_64)
+            ;;
+        arm64)
+            brew install tidy-html5
+            ;;
+        *)
+            echo "unsupported architecture: ${platform}"
+            exit 1
+    esac
+fi