Avoid giving Pathname#relative_path_from a String for Ruby < 2.6 compat

[amatsuda]

rake-compiler started to fail on Ruby 2.5 since its version 1.2.4. Here's an actual failure that I saw on haml project, https://github.com/haml/haml/actions/runs/5740494510/job/15558470282#step:5:26
which fails with NoMethodError: undefined method `cleanpath' for "tmp/x86_64-linux/haml/2.5.9":String.

The error seems to be raised from this line

rake-compiler/lib/rake/extensiontask.rb

Line 166 in 77d9294

relative_lib_path = Pathname(lib_path).relative_path_from(tmp_path)

and it can be blamed to 0d93b08.

This happens because Pathname#relative_path_from used not to accept a String parameter until ruby/ruby#2049 (ruby/ruby@4cf8286 and ruby/ruby@78dc3da).

The CI on this repo is passing on Ruby 2.5 although it's not working, because there are no test that tries to run copy:* task, whereas there exist some tests that test the definition or dependency to that task. I'm not sure if I'd better add such a test, but for now, let me just post a minimum patch that should fix the error.

[kou]

Thanks!
I've merged #226 instead of this because #226 submitted before this. Sorry.

[amatsuda]

Oh, thank you @ivoanjo and @flavorjones! I should have checked the existing issue reports before investigating.