build: improve cross compilation setup (#22804)

This PR cleans up the CI build system and fixes a couple of issues.

- The go tool launcher code has been moved to internal/build. With the new
  toolchain functions, the environment of the host Go (i.e. the one that built
  ci.go) and the target Go (i.e. the toolchain downloaded by -dlgo) are isolated
  more strictly. This is important to make cross compilation and -dlgo work
  correctly in more cases.
- The -dlgo option now skips the download and uses the host Go if the running Go
  version matches dlgoVersion exactly.
- The 'test' command now supports -dlgo, -cc and -arch. Running unit tests with
  foreign GOARCH is occasionally useful. For example, it can be used to run
  32-bit tests on Windows. It can also be used to run darwin/amd64 tests on
  darwin/arm64 using Rosetta 2.
- The 'aar', 'xcode' and 'xgo' commands now use a slightly different method to
  install external tools. They previously used `go get`, but this comes with the
  annoying side effect of modifying go.mod. They now use `go install` instead,
  which is the recommended way of installing tools without modifying the local
  module.
- The old build warning about outdated Go version has been removed because we're
  much better at keeping backwards compatibility now.

internal/build/util.go

@@ -29,7 +29,6 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"text/template"
 )
@@ -111,19 +110,6 @@ func render(tpl *template.Template, outputFile string, outputPerm os.FileMode, x
 	}
 }
 
-// GoTool returns the command that runs a go tool. This uses go from GOROOT instead of PATH
-// so that go commands executed by build use the same version of Go as the 'host' that runs
-// build code. e.g.
-//
-//     /usr/lib/go-1.12.1/bin/go run build/ci.go ...
-//
-// runs using go 1.12.1 and invokes go 1.12.1 tools from the same GOROOT. This is also important
-// because runtime.Version checks on the host should match the tools that are run.
-func GoTool(tool string, args ...string) *exec.Cmd {
-	args = append([]string{tool}, args...)
-	return exec.Command(filepath.Join(runtime.GOROOT(), "bin", "go"), args...)
-}
-
 // UploadSFTP uploads files to a remote host using the sftp command line tool.
 // The destination host may be specified either as [user@]host[: or as a URI in
 // the form sftp://[user@]host[:port].