fix(build.sh): name macOS files after the darwin platform

This was what we used to do in the past. Spotted by @sarathms.

.github/workflows/macos.yml

@@ -12,9 +12,9 @@ jobs:
         with:
           go-version: "1.14"
       - uses: actions/checkout@v2
-      - run: ./build.sh macos
-      - run: ./smoketest.sh ./CLI/macos/amd64/ooniprobe
+      - run: ./build.sh darwin
+      - run: ./smoketest.sh ./CLI/darwin/amd64/ooniprobe
       - uses: actions/upload-artifact@v1
         with:
-          name: ooniprobe-macos-amd64
-          path: ./CLI/macos/amd64/ooniprobe
+          name: ooniprobe-darwin-amd64
+          path: ./CLI/darwin/amd64/ooniprobe