Download correct Bazel binary on Linux (bazelbuild#346)

We stopped publishing binaries for Ubuntu 14.04 some time ago, and we don't upload Arm64 Linux binaries (yet).
Consequently, Bazelisk on Linux should always fetch the regular Ubuntu 18.04 binaries for now.

Fixes bazelbuild/bazel#16061

platforms/platforms.go

@@ -3,22 +3,41 @@ package platforms
 
 import (
 	"fmt"
-	semver "github.com/hashicorp/go-version"
 	"log"
 	"runtime"
+
+	semver "github.com/hashicorp/go-version"
 )
 
-var platforms = map[string]string{"darwin": "macos", "linux": "ubuntu1404", "windows": "windows"}
+type platform struct {
+	Name           string
+	HasArm64Binary bool
+}
+
+var supportedPlatforms = map[string]*platform{
+	"darwin": {
+		Name:           "macos",
+		HasArm64Binary: true,
+	},
+	"linux": {
+		Name:           "ubuntu1804",
+		HasArm64Binary: false,
+	},
+	"windows": {
+		Name:           "windows",
+		HasArm64Binary: true,
+	},
+}
 
 // GetPlatform returns a Bazel CI-compatible platform identifier for the current operating system.
 // TODO(fweikert): raise an error for unsupported platforms
 func GetPlatform() string {
-	platform := platforms[runtime.GOOS]
+	platform := supportedPlatforms[runtime.GOOS]
 	arch := runtime.GOARCH
-	if arch == "arm64" {
-		platform = platform + "_arm64"
+	if arch == "arm64" && platform.HasArm64Binary {
+		return platform.Name + "_arm64"
 	}
-	return platform
+	return platform.Name
 }
 
 // DetermineExecutableFilenameSuffix returns the extension for binaries on the current operating system.