Installing twice to the same location can cause Permission denied errors

[eregon]

For example:
(I'm running this after my fixes for #522)

$ bin/ruby-install truffleruby-graalvm-33.0.0 --prefix /tmp/truffleruby
works fine
$ bin/ruby-install truffleruby-graalvm-33.0.0 --prefix /tmp/truffleruby
...
>>> Installing TruffleRuby GraalVM 33.0.0 ...
cp: cannot create regular file '/tmp/truffleruby/jvm/legal/jdk.sctp/LICENSE': Permission denied
cp: cannot create regular file '/tmp/truffleruby/jvm/legal/jdk.sctp/COPYRIGHT': Permission denied
cp: cannot create regular file '/tmp/truffleruby/jvm/legal/jdk.crypto.ec/LICENSE': Permission denied
cp: cannot create regular file '/tmp/truffleruby/jvm/legal/jdk.crypto.ec/COPYRIGHT': Permission denied
cp: cannot create regular file '/tmp/truffleruby/jvm/legal/jdk.incubator.vector/LICENSE': Permission denied
...

The reason is those files have -r--r--r--. permissions.

Naturally, rm -rf /tmp/truffleruby between both commands fixes it.

@postmodern What do you think would be a good solution here?

I'm thinking rm -rf $install_dir might be best, and avoid e.g. potentially mixing old and new files.