x/build/internal/installer/{darwinpkg,windowsmsi}: report if GOTOOLCHAIN in environment is set to an older version than one being installed

[dmitshur]

When an installer for a certain Go version runs, it can check if the environment is configured such that go version would select select an older toolchain than the one being installed, and potentially highlight that to the user. (Another option may be to modify GOTOOLCHAIN in environment, either automatically or after prompting, but this might be trickier to get right.)

For example, suppose someone installs Go 1.22.0 with an installer, then later upgrades to Go 1.22.1 by setting go env -w GOTOOLCHAIN=go1.22.1+auto, then later on uses the installer once again to install a Go 1.22.2 local toolchain. Unless they change the GOTOOLCHAIN value in the environment, the installer would be a no-op, so it might be a good thing to report to the user, in case it was unintentional on their part.

(The macOS installer already has a few checks at https://cs.opensource.google/go/x/build/+/master:internal/installer/darwinpkg/_data/dist.xml;l=17-28;drc=0ce25ffd88baa2eb34517f5c0528e4a79d5e924f.)

Thanks to @hyangah for suggesting this.