Revert "ci: update go to 1.18"

This reverts commit c8e93d2.

golangci-lint does not support 1.18 yet
(golangci/golangci-lint#2649), postpone the upgrade.

Author: Fabian Holler

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Install Go
         uses: actions/setup-go@v2
         with:
-          go-version: 1.18
+          go-version: 1.17
 
       - name: Check out code
         uses: actions/checkout@v2
@@ -29,7 +29,7 @@ jobs:
       - name: Install Go
         uses: actions/setup-go@v2
         with:
-          go-version: 1.18
+          go-version: 1.17
 
       - name: Check out code
         uses: actions/checkout@v2