go.mod: update x/tools to HEAD

go get -u golang.org/x/tools
go mod tidy

x/tools/go/ssa is used by message/pipeline. Generics instantiation in
x/[email protected] could be confused with container index. However,
x/tools/go/ssa does have it fixed in x/[email protected] (see golang/go#52834).
This change upgrades the required version to ensure that fewer users of
message/pipelines have issues.

Updates golang/go#54086

Author: yarcat