ci: prepare for golangci-lint-action v3

It needs golang to be installed (if the default golang which comes
preinstalled in Ubuntu is not new/good enough).

In our case, we need at least go 1.16, and can use 1.17 or 1.18 (when
it is released), but we don't set it explicitly here to ease the
maintenance burden (1.x should point to the latest stable).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/validate.yml

@@ -14,6 +14,9 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/setup-go@v2
+        with:
+          go-version: 1.x # Latest stable
       - name: install deps
         run: |
           sudo apt -q update
@@ -30,6 +33,9 @@ jobs:
       contents: read
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/setup-go@v2
+        with:
+          go-version: 1.x # Latest stable
       - name: install deps
         run: |
           sudo apt -q update