Support multiple benchmark packages per macOS workflow invocation

[rnro]

Motivation

• External callers of macos_benchmarks.yml (e.g. swift-log) currently invoke the reusable workflow once per benchmark package, paying self-hosted macOS runner-recycling cost for each invocation.

Modifications

• Add a new benchmark_package_paths input to .github/workflows/macos_benchmarks.yml and mark the singular benchmark_package_path as deprecated. The run step forwards both inputs straight to the script via BENCHMARK_PACKAGE_PATH and BENCHMARK_PACKAGE_PATHS environment variables; the workflow itself does no path parsing.
• Teach scripts/check_benchmark_thresholds.sh to accept BENCHMARK_PACKAGE_PATHS as either a JSON array of strings (auto-detected by a leading [, requires jq) or a newline-separated list, falling back to the legacy BENCHMARK_PACKAGE_PATH when unset or when the JSON form is an empty array.
• The script now loops over the resolved package list, wraps each iteration in a GitHub Actions ::group::, and emits an ::error:: summary listing failed packages at the end, exiting non-zero if any failed.

Result

• A single workflow invocation runs all listed packages serially on each Swift/Xcode runner, paying runner-recycling cost once per Swift version instead of once per (version x package).
• Existing single-package callers keep working unchanged.
• Adopters of the script in other contexts (e.g. the Linux benchmarks.yml) can pick up multi-package support by passing BENCHMARK_PACKAGE_PATHS without reimplementing parsing or failure-collection logic.

An example of this working apple/swift-log#466