go run supports running an arbitrary go modules like this:
go:generate go run golang.org/x/tools/cmd/[email protected]

By using that everywhere that our build wanted to demand a previous
install of some program, we don't need any of the complexity around
`check_deps.sh`, `install_buildtools.sh` and the like.  This PR
removes them.

There shouldn't be a performance problem because the programs will run
out of Go's local module cache.  Let's see.

Co-authored-by: ohill <[email protected]>