Force golangci-lint to run on every supported Os (elastic#234)

* Force golangci-lint to run on every supported Os

By default `golangci-lint` will run the linters for the current running
platform and will only include the files for this platform. Using the `GOOS`
environment variable and a matrix defined in the github action we force
the linter to try all the supported operating system.

.github/workflows/golangci-lint.yml

@@ -12,9 +12,20 @@ permissions:
   pull-requests: read
 jobs:
   golangci:
+    strategy:
+      matrix:
+        include:
+          - GOOS: windows
+          - GOOS: linux
+          - GOOS: darwin
     name: lint
     runs-on: ubuntu-latest
     steps:
+      - name: Echo details
+        env:
+           GOOS: ${{ matrix.GOOS }}
+        run: echo Go GOOS=$GOOS
+
       - uses: actions/checkout@v2
 
       # Uses Go version from the repository.
@@ -27,6 +38,8 @@ jobs:
           go-version: "${{ steps.goversion.outputs.version }}"
 
       - name: golangci-lint
+        env:
+          GOOS: ${{ matrix.GOOS }}
         uses: golangci/golangci-lint-action@v2
         with:
           # Optional: version of golangci-lint to use in form of v1.2 or v1.2.3 or `latest` to use the latest version