ci: run package bumps as parallel steps

.github/scripts/bump-zshellcheck.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# Updates Formula/zshellcheck.rb to the latest zshellcheck release.
+# On a new version it writes "$BUMP_DIR/zshellcheck": the commit message, then the changed files.
+set -euo pipefail
+formula=Formula/zshellcheck.rb
+repo=afadesigns/zshellcheck
+latest=$(gh api repos/$repo/releases/latest --jq .tag_name)
+latest=${latest#v}
+current=$(sed -nE 's|.*/releases/download/v([^/]+)/.*|\1|p' "$formula" | head -1)
+if [ "$latest" = "$current" ]; then
+  echo "Already at $current"
+  exit 0
+fi
+
+# The release publishes checksums.txt with a sha256 for every asset.
+if ! checksums=$(curl -fsSL "https://github.com/$repo/releases/download/v$latest/checksums.txt"); then
+  echo "checksums.txt is not published yet"
+  exit 0
+fi
+declare -A sha
+for asset in zshellcheck_Darwin_{arm64,x86_64}.tar.gz; do
+  sha[$asset]=$(awk -v a="$asset" '$2 == a { print $1 }' <<<"$checksums")
+  if [ -z "${sha[$asset]}" ]; then
+    echo "$asset is not published yet"
+    exit 0
+  fi
+done
+
+sed -i -E "s|/v${current//./\\.}/|/v$latest/|g" "$formula"
+for asset in "${!sha[@]}"; do
+  sed -i -E "/\/${asset//./\\.}\"/{n;s/[0-9a-f]{64}/${sha[$asset]}/}" "$formula"
+done
+echo "chore: bump zshellcheck to $latest" > "$BUMP_DIR/zshellcheck"
+echo "$formula" >> "$BUMP_DIR/zshellcheck"