github/workflows: make sure we don't break go1.15 builds

We now have a workaround for people building using go1.15. We wanna make
sure with a simple smoke test we produce a working miniooni binary.

Also part of #866.

.github/workflows/go115.yml

@@ -0,0 +1,18 @@
+name: go115
+on:
+  pull_request:
+  schedule:
+    - cron: "14 17 * * 3"
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/setup-go@v1
+        with:
+          go-version: "1.15"
+      - uses: actions/checkout@v2
+      - run: ./build-cli.sh linux
+      - run: ./CLI/linux/amd64/miniooni -nNi https://example.com web_connectivity
+      - run: ./build-cli.sh darwin
+      - run: sudo apt install --yes mingw-w64
+      - run: ./build-cli.sh windows

.github/workflows/miniooni.yml

@@ -13,6 +13,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - run: ./build-cli.sh linux
+      - run: ./CLI/linux/amd64/miniooni -nNi https://example.com web_connectivity
       - uses: actions/upload-artifact@v1
         with:
           name: miniooni-linux-amd64