Update go version to 1.18.4

Over in gitlab-org/gitlab-runner!3524 (closed), as part of upgrading gitlab-runner's go version to 1.18, we also updated the version of golangci-lint. When using go's plugin functionality, for any dependencies used both in the plugin and the code consuming the plugin, the versions of said dependencies must be the same in the plugin and the code consuming the plugin. All this to say that the version of golang.org/x/tools used in this here plugin must be the same version as is used in golangci-lint (which consumes this plugin). This MR makes this so, and also upgrades the go version (and does some other cleanups) in the process.