fix(msys2): detect RubyInstaller correctly

[yumetodo]

On pure msys2 mingw64 environment, The code like below will fail:

require 'ruby2d'
show

$ruby test.rb
Traceback (most recent call last):
        2: from test.rb:1:in `<main>'
        1: from C:/msys64/mingw64/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
C:/msys64/mingw64/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require': cannot load such file -- ruby2d (LoadError)
        4: from test.rb:1:in `<main>'
        3: from C:/msys64/mingw64/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:34:in `require'
        2: from C:/msys64/mingw64/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:130:in `rescue in require'
        1: from C:/msys64/mingw64/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:130:in `require'
C:/msys64/mingw64/lib/ruby/gems/2.6.0/gems/ruby2d-0.9.2/lib/ruby2d.rb:25:in `<top (required)>': uninitialized constant RubyInstaller (NameError)

Because RUBY_PLATFORM =~ /mingw/ also match with pure msys2 mingw64 environment.

this bug was cause by d796435 .

ref:

• Add RubyInstaller2 support #91
• https://teratail.com/questions/202950

---

note

Currently, rake test:native audio will fail due to another issue(usage of File.foreach). This PR doesn't resolve this issue( #167 ) .