Add "go" _util command; use it to run getmingw

[dagood]

Add pwsh eng/run.ps1 go [...]. This runs any go command, but uses the stage 0 version of Go, which is the Microsoft build of Go.

Use it during the build to run github.com/microsoft/go-infra/cmd/getmingw rather than using an ambiently installed go.

This is also useful in general: it's sometimes annoying to work with the _util module because it doesn't live in the root of the repo. Now you can use commands like pwsh eng/run.ps1 go mod tidy from the root to maintain the _util module.

• Should fix the build cache issue in build(deps): bump golang.org/x/net from 0.50.0 to 0.51.0 in /eng/_util #2162, but it's the right thing to do regardless.

[Copilot]

Pull request overview

This PR adds a new _util tool (go) that runs go commands using the stage 0 (Microsoft) Go toolchain, and updates the Windows pipeline to use it for running getmingw so builds don’t depend on an ambient Go installation.

Changes:

• Add eng/_util/cmd/go tool to proxy go invocations through the stage 0 toolchain and run from the _util module directory.
• Update the Windows pipeline MinGW install step to run github.com/microsoft/go-infra/cmd/getmingw via eng/run.ps1 go run ... instead of go install + GOPATH binary execution.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
eng/pipeline/stages/run-stage.yml	Switch MinGW installation to use the new stage0-backed go tool via eng/run.ps1.
eng/_util/cmd/go/go.go	New _util command that runs stage0 go with working directory set to eng/_util.