fix: include windows platform and use filter_map

[BSFishy]

I had bundix start failing for me with this error:

/nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/nixer.rb:100:in 'Bundix::Nixer#serialize': Cannot convert to nix: nil (RuntimeError)
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/nixer.rb:69:in 'Bundix::Nixer#sub'
        from (erb):1:in 'block in Bundix::Nixer#serialize'
        from (erb):1:in 'Array#each'
        from (erb):1:in 'Enumerable#each_with_index'
        from (erb):1:in 'Bundix::Nixer#serialize'
        from /nix/store/wwgdz0w3i4xyn2rxd73m1gv84695sa90-ruby-3.4.7/lib/ruby/3.4.0/erb.rb:429:in 'Kernel#eval'
        from /nix/store/wwgdz0w3i4xyn2rxd73m1gv84695sa90-ruby-3.4.7/lib/ruby/3.4.0/erb.rb:429:in 'ERB#result'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/nixer.rb:85:in 'Bundix::Nixer#serialize'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/nixer.rb:69:in 'Bundix::Nixer#sub'
        from (erb):2:in 'block in Bundix::Nixer#serialize'
        from (erb):2:in 'Array#each'
        from (erb):2:in 'Bundix::Nixer#serialize'
        from /nix/store/wwgdz0w3i4xyn2rxd73m1gv84695sa90-ruby-3.4.7/lib/ruby/3.4.0/erb.rb:429:in 'Kernel#eval'
        from /nix/store/wwgdz0w3i4xyn2rxd73m1gv84695sa90-ruby-3.4.7/lib/ruby/3.4.0/erb.rb:429:in 'ERB#result'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/nixer.rb:83:in 'Bundix::Nixer#serialize'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/nixer.rb:69:in 'Bundix::Nixer#sub'
        from (erb):2:in 'block in Bundix::Nixer#serialize'
        from (erb):2:in 'Array#each'
        from (erb):2:in 'Bundix::Nixer#serialize'
        from /nix/store/wwgdz0w3i4xyn2rxd73m1gv84695sa90-ruby-3.4.7/lib/ruby/3.4.0/erb.rb:429:in 'Kernel#eval'
        from /nix/store/wwgdz0w3i4xyn2rxd73m1gv84695sa90-ruby-3.4.7/lib/ruby/3.4.0/erb.rb:429:in 'ERB#result'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/nixer.rb:83:in 'Bundix::Nixer#serialize'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/nixer.rb:20:in 'Bundix::Nixer.serialize'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/commandline.rb:153:in 'Bundix::CommandLine#object2nix'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/commandline.rb:159:in 'Bundix::CommandLine#save_gemset'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/commandline.rb:37:in 'Bundix::CommandLine#run'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/lib/bundix/commandline.rb:22:in 'Bundix::CommandLine.run'
        from /nix/store/wqsqmnmnp3rzarnc6acdnnwvs4cv87sc-bundix/bin/bundix:5:in '<main>'

I tracked it down to a missing windows platform, which is now added in this PR. I also changed the map to a filter_map to prevent something like this happening in the future, but can remove if that's not desirable