Avoid "permission denied" errors copying assets

[tomeon]

by running FileUtils#cp_r with the :remove_destination option enabled. This makes asset copying more robust in the face of restrictive permissions on asset file destination paths. This should be safe and backward-compatible given that the destination files (or their contents) will be fully replaced anyway.

Example case:

irb(main):001:0> FileUtils.touch('foo')
=> ["foo"]
irb(main):002:0> FileUtils.touch('bar')
=> ["bar"]
irb(main):003:0> FileUtils.chmod(0o444, 'bar')
=> ["bar"]
irb(main):004:0> FileUtils.cp_r('foo', 'bar')
Errno::EACCES: Permission denied @ rb_sysopen - bar from /nix/store/2kw126cy93rix4pmh9lcl120njnb6r7r-ruby-3.1.4/lib/ruby/3.1.0/fileutils.rb:1395:in `initialize' 
irb(main):005:0> FileUtils.cp_r('foo', 'bar', remove_destination: true) 
=> nil

Addresses simplecov-ruby/simplecov#741.

Thanks!

[tomeon]

Note that this should solve #133.