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- The old build warning about outdated Go version has been removed because we're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/env.go

@@ -38,6 +38,7 @@ var (
 
 // Environment contains metadata provided by the build environment.
 type Environment struct {
+	CI                        bool
 	Name                      string // name of the environment
 	Repo                      string // name of GitHub repo
 	Commit, Date, Branch, Tag string // Git info
@@ -61,6 +62,7 @@ func Env() Environment {
 			commit = os.Getenv("TRAVIS_COMMIT")
 		}
 		return Environment{
+			CI:            true,
 			Name:          "travis",
 			Repo:          os.Getenv("TRAVIS_REPO_SLUG"),
 			Commit:        commit,
@@ -77,6 +79,7 @@ func Env() Environment {
 			commit = os.Getenv("APPVEYOR_REPO_COMMIT")
 		}
 		return Environment{
+			CI:            true,
 			Name:          "appveyor",
 			Repo:          os.Getenv("APPVEYOR_REPO_NAME"),
 			Commit:        commit,