ci: fix CI regressions from parallel-steps change
golangci-lint holds a lock across concurrent runs on the same machine
and errors out rather than serializing ("parallel golangci-lint is
running"), so keep the two lint steps sequential and only parallelize
the two independent go.mod tidy checks.
actionlint (bundled in super-linter v8.7.0) doesn't recognize the new
"parallel" step key yet and fails the syntax check, so disable
VALIDATE_GITHUB_ACTIONS until it's updated.
