github: add workflow to test on macos

We have some users on macOS. builds.sr.ht does not support proprietary
build images and will never do.

Signed-off-by: Robin Jarry <robin@jarry.cc>

.github/workflows/macos.yml

@@ -0,0 +1,16 @@
+---
+on: push
+
+jobs:
+  macos:
+    runs-on: macos-latest
+    env:
+      DESTDIR: ./out
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-go@v2
+      - run: brew install scdoc
+      - run: make
+      - run: make install
+      - run: make checkinstall
+      - run: go test ./...