Use the path returned from Gem::Source#download

`Gem::SpecFetcher.fetcher.spec_for_dependency` returns
`Gem::Specification`, `Gem::Source` tuples.

https://github.com/fedora-ruby/gem-compare/blob/6b221bcf37b30a9dda05d0a2d7d677bee52ef097/lib/rubygems/comparator.rb#L251

```ruby
irb(main):001> gem_name = "pg"
=> "pg"
irb(main):002> version = "1.6.0"
=> "1.6.0"
irb(main):003> dep = Gem::Dependency.new gem_name, version
=> Gem::Dependency.new("pg", Gem::Requirement.new(["= 1.6.0"]), :runtime)
irb(main):004> specs_and_sources, _errors = Gem::SpecFetcher.fetcher.spec_for_dependency dep
=>
[[[Gem::Specification.new do |s|
...
irb(main):017> specs_and_sources.map { it.map(&:class) }
=> [[Gem::Specification, Gem::Source], [Gem::Specification, Gem::Source]]
```

`Gem::Source#download` will return the local gem file path:

```ruby
irb(main):013> specs_and_sources.first.last.download specs_and_sources.first.first
Fetching pg-1.6.0-aarch64-linux.gem
=> "/app/pg-1.6.0-aarch64-linux.gem"
```

Author: dentarg

lib/rubygems/comparator.rb

@@ -225,15 +225,14 @@ def get_package(gem_name, version)
 
     def download_package(gem_name, version)
       spec, source = get_specification(gem_name, version)
-      gem_file = gem_file_name(gem_name, spec.version.to_s)
 
-      Dir.chdir @options[:output] do
+      gem_file_path = Dir.chdir @options[:output] do
         source.download spec
       end
 
-      package = Gem::Package.new File.join(@options[:output], gem_file)
+      package = Gem::Package.new(gem_file_path)
       use_package(package)
-      info "#{gem_file} downloaded."
+      info "#{gem_file_path} downloaded."
 
       package
     end