Avoid "permission denied" errors copying assets

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.

lib/simplecov-html.rb

@@ -27,7 +27,7 @@ def initialize
       def format(result)
         unless @inline_assets
           Dir[File.join(@public_assets_dir, "*")].each do |path|
-            FileUtils.cp_r(path, asset_output_path)
+            FileUtils.cp_r(path, asset_output_path, remove_destination: true)
           end
         end