github/workflows/main: only run staticcheck on the lastest version

Tests and build should work on both supported versions, but we can lint
only the most recent one.

Author: fsouza

.github/workflows/main.yml

@@ -62,6 +62,7 @@ jobs:
       - run: go mod download
 
       - run: make staticcheck
+        if: matrix.go_version == '1.18'
 
       - run: make gotest
         env: