Simplify `truffleruby+graalvm-dev` build definition re: download redirect

[mislav]

The http get <url> <destfile> utility had a bug with aria2c downloader where it couldn't properly save to destfile if it was an absolute path.

I have tried having http get <url> - consistently print the downloaded file to stdout, but this conflicted with the output of log_command (which is also to stdout) so for now let's keep using the temporary file to resolve manual URL redirects.

Followup to https://github.com/rbenv/ruby-build/pull/2356 /cc @rwstauner