Ensure it is safe to remove the prefix path for TruffleRuby or error

[eregon]

• Only do clean_prefix_path for TruffleRuby and rename for clarity,
  since the logic is now TruffleRuby-specific.
• Alternative to Stop deleting the prefix path on install of TruffleRuby #1783, cc @casperisfine

The error message looks like:

$ bin/ruby-build truffleruby-dev /usr/local
Downloading truffleruby-head-ubuntu-18.04.tar.gz...
-> https://github.com/ruby/truffleruby-dev-builder/releases/latest/download/truffleruby-head-ubuntu-18.04.tar.gz
Installing truffleruby-head...

ERROR: cannot install TruffleRuby to /usr/local, which does not look like a valid TruffleRuby prefix
TruffleRuby only supports being installed to a not existing directory or replacing an existing TruffleRuby installation
See https://github.com/oracle/truffleruby/issues/1389 for details

BUILD FAILED (OS VERSION using ruby-build 20210720-2-g6fa319f)

Inspect or clean up the working tree at /tmp/ruby-build.20210726155937.184076.UFViZv
Results logged to /tmp/ruby-build.20210726155937.184076.log