Update build of x64 macos binary (#1518)

Looks like `macos-latest` is changing to arm64 so we can't rely on it
implicitly building x86_64-apple-darwin, so explicitly build it instead.

Author: alexcrichton

.github/workflows/main.yml

@@ -24,6 +24,7 @@ jobs:
           os: ubuntu-latest
         - build: x86_64-macos
           os: macos-latest
+          target: x86_64-apple-darwin
         - build: aarch64-macos
           os: macos-latest
           target: aarch64-apple-darwin