Support building on arm macs

Attempts to pull down butler were failing, since there's no arm64
channel available. Instead, we force all macs to fetch the amd64
channel.

src/main/broth/package.ts

@@ -14,11 +14,11 @@ import * as sf from "main/os/sf";
 import { mkdir, readdir } from "main/os/sf";
 import { delay } from "main/reactors/delay";
 import formulas, { FormulaSpec } from "main/broth/formulas";
-import { goarch, goos } from "main/broth/platform";
+import { platformString } from "main/broth/platform";
 import { unzip } from "main/broth/unzip";
 
 const sanityCheckTimeout = 10000;
-const platform = `${goos()}-${goarch()}`;
+const platform = platformString();
 
 const downloadStart = 0.0;
 const downloadWeight = 0.3;

src/main/broth/platform.ts

@@ -1,7 +1,7 @@
 import { arch } from "main/os/arch";
 
 /** platform in go format */
-export function goos(): string {
+function goos(): string {
   let result = process.platform;
   if (result === "win32") {
     return "windows";
@@ -10,7 +10,7 @@ export function goos(): string {
 }
 
 /** arch in go format */
-export function goarch() {
+function goarch() {
   let result = arch();
   if (result === "x64") {
     return "amd64";
@@ -20,3 +20,14 @@ export function goarch() {
     return "unknown";
   }
 }
+
+export function platformString(): string {
+  const os = goos();
+  let arch: string;
+  if (os === "darwin") {
+    arch = "amd64";
+  } else {
+    arch = goarch();
+  }
+  return `${os}-${arch}`;
+}