Introduce tool directive in `go.mod`

[sue445]

ref. #417

Run go install golang.org/x/tools/cmd/goimports@latest
go: downloading golang.org/x/tools v0.43.0
go: golang.org/x/tools/cmd/goimports@latest: golang.org/x/tools@v0.43.0 requires go >= 1.25.0 (running go 1.24.13; GOTOOLCHAIN=local)

https://github.com/ruby-go-gem/go-gem-wrapper/actions/runs/23164241503/job/67299909431?pr=417

goimports@latest requires Go 1.25+, but I want to support Go 1.24 yet.