Download native macOS arm64 binary on M1 (bazelbuild#303)

* Download native macOS arm64 binary on M1

* Move logic to platforms.go

* Support declared platforms with arm64

* Minor fixes

platforms/platforms.go

@@ -13,7 +13,12 @@ var platforms = map[string]string{"darwin": "macos", "linux": "ubuntu1404", "win
 // GetPlatform returns a Bazel CI-compatible platform identifier for the current operating system.
 // TODO(fweikert): raise an error for unsupported platforms
 func GetPlatform() string {
-	return platforms[runtime.GOOS]
+	platform := platforms[runtime.GOOS]
+	arch := runtime.GOARCH
+	if arch == "arm64" {
+		platform = platform + "_arm64"
+	}
+	return platform
 }
 
 // DetermineExecutableFilenameSuffix returns the extension for binaries on the current operating system.

test.sh

@@ -3,13 +3,15 @@
 set -euxo pipefail
 
 rm -rf "$HOME/Library/Caches/bazelisk"
-env -u USE_BAZEL_VERSION ./bin/bazelisk-darwin-amd64 version
-USE_BAZEL_VERSION="latest" ./bin/bazelisk-darwin-amd64 version
+
+arch=$(uname -m)
+env -u USE_BAZEL_VERSION ./bin/bazelisk-darwin-"$arch" version
+USE_BAZEL_VERSION="latest" ./bin/bazelisk-darwin-"$arch" version
 USE_BAZEL_VERSION="0.28.0" ./bin/bazelisk-darwin-amd64 version
-USE_BAZEL_VERSION="last_green" ./bin/bazelisk-darwin-amd64 version
-USE_BAZEL_VERSION="last_downstream_green" ./bin/bazelisk-darwin-amd64 version
-USE_BAZEL_VERSION="last_rc" ./bin/bazelisk-darwin-amd64 version
-USE_BAZEL_VERSION="bazelbuild/latest" ./bin/bazelisk-darwin-amd64 version
+USE_BAZEL_VERSION="last_green" ./bin/bazelisk-darwin-"$arch" version
+USE_BAZEL_VERSION="last_downstream_green" ./bin/bazelisk-darwin-"$arch" version
+USE_BAZEL_VERSION="last_rc" ./bin/bazelisk-darwin-"$arch" version
+USE_BAZEL_VERSION="bazelbuild/latest" ./bin/bazelisk-darwin-"$arch" version
 USE_BAZEL_VERSION="bazelbuild/0.27.0" ./bin/bazelisk-darwin-amd64 version
-USE_BAZEL_VERSION="philwo/latest" ./bin/bazelisk-darwin-amd64 version
+USE_BAZEL_VERSION="philwo/latest" ./bin/bazelisk-darwin-"$arch" version
 USE_BAZEL_VERSION="philwo/0.25.0" ./bin/bazelisk-darwin-amd64 version