CI: allow Go 1.13 for Docker/Moby compatibility

Docker/Moby still uses Go 1.13 for building containerd binaries.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.github/workflows/ci.yml

@@ -217,11 +217,16 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-18.04, macos-10.15, windows-2019]
+        go-version: ['1.16.3']
+        include:
+          # Go 1.13.x is still used by Docker/Moby
+          - go-version: '1.13.x'
+            os: ubuntu-18.04
 
     steps:
       - uses: actions/setup-go@v2
         with:
-          go-version: '1.16.3'
+          go-version: ${{ matrix.go-version }}
 
       - name: Set env
         shell: bash
@@ -571,4 +576,4 @@ jobs:
           name:  TestResults cgroup2 ${{ matrix.runtime }} ${{matrix.runc}}
           path: |
             ${{github.workspace}}/*-junit.xml
-            ${{github.workspace}}/critestreport/*
+            ${{github.workspace}}/critestreport/*