If the user has the encoding of her system messed up, she may end up
sending us incorrectly encoding input, causing "invalid byte sequence in
UTF-8" errors at random places.

These errors can be forced on a system without encoding issues with
something like:

```
$ gem install$(echo -e "\xFF") foo
/Users/deivid/.asdf/installs/ruby/3.2.1/lib/ruby/site_ruby/3.2.0/rubygems/config_file.rb:534:in `block in set_config_file_name': invalid byte sequence in UTF-8 (ArgumentError)
	from /Users/deivid/.asdf/installs/ruby/3.2.1/lib/ruby/site_ruby/3.2.0/rubygems/config_file.rb:530:in `each'
	from /Users/deivid/.asdf/installs/ruby/3.2.1/lib/ruby/site_ruby/3.2.0/rubygems/config_file.rb:530:in `set_config_file_name'
	from /Users/deivid/.asdf/installs/ruby/3.2.1/lib/ruby/site_ruby/3.2.0/rubygems/config_file.rb:177:in `initialize'
	from /Users/deivid/.asdf/installs/ruby/3.2.1/lib/ruby/site_ruby/3.2.0/rubygems/gem_runner.rb:71:in `new'
	from /Users/deivid/.asdf/installs/ruby/3.2.1/lib/ruby/site_ruby/3.2.0/rubygems/gem_runner.rb:71:in `do_configuration'
	from /Users/deivid/.asdf/installs/ruby/3.2.1/lib/ruby/site_ruby/3.2.0/rubygems/gem_runner.rb:33:in `run'
	from /Users/deivid/.asdf/installs/ruby/3.2.1/bin/gem:10:in `<main>'
```

This commit makes RubyGems print a better error in this case:

```
$ ruby -Ilib bin/gem install$(echo -e "\xFF") foo
/Users/deivid/Code/rubygems/rubygems/lib/rubygems/gem_runner.rb:75:in `validate_encoding': invalid argument: 'install�' has invalid encoding (Gem::OptionParser::InvalidArgument)
	from /Users/deivid/Code/rubygems/rubygems/lib/rubygems/gem_runner.rb:31:in `run'
	from bin/gem:10:in `<main>'
```