Compiler-only build instructions build in a version-specific dir

The idea was the instructions would build in `~/rakudo/`, but they actually build in `~/rakudo/rakudo-*/`.

Also, is there a point in curling and untarring an archive rather than just `git`-ting from a repo (and doing `git checkout $(git describe --abbrev=0 --tags)` to checkout last release) , since NQP/MoarVM are fetched from a repo anyway?