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- The old build warning about outdated Go version has been removed because we're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