github/workflows/main: add cross compilation targets for arm64

GitHub Actions doesn't offer hosted nodes with arm64 yet, so we can't
run tests, but let's at least try to compile it (similar to freebsd).

Author: fsouza

.github/workflows/main.yml

@@ -18,8 +18,11 @@ jobs:
           - freebsd
           - linux
           - windows
+        goarch:
+          - amd64
+          - arm64
 
-    name: cross-compilation (GOOS=${{ matrix.goos }})
+    name: cross-compilation (GOOS=${{ matrix.goos }}, GOARCH=${{ matrix.goarch }})
     runs-on: ubuntu-latest
     steps:
       - uses: actions/[email protected]
@@ -31,6 +34,7 @@ jobs:
       - run: go build
         env:
           GOOS: ${{ matrix.goos }}
+          GOARCH: ${{ matrix.goarch }}
 
   test:
     strategy: