cmd/go: improve "toolchain not available" message when patch version is missing #66175
Description
xtools$ GOTOOLCHAIN=go1.22 go run hello.go
go: downloading go1.22 (darwin/arm64)
go: download go1.22 for darwin/arm64: toolchain not available
xtools$ GOTOOLCHAIN=go1.22.0 go run hello.go
Hello, world.
It took me a long while to figure out that the cause of the problem was a missing patch version suffix (.0). Either the go command should pick a reasonable suffix itself, or it should report that I am expected to provide one.