FlutterでCocoaPodsインストール時のエラー対処方法

FlutterでiOSの開発環境構築時に少しエラーが出てハマったので、その時の対処方法を紹介します。

環境

macOS：10.14.6
Ruby：2.3.7p456 (2018-03-28 revision 63024)
Flutter SDK：1.20.1

現象

CocoaPodsをインストールしようとするとエラーが発生する。

$ sudo gem install cocoapods
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/universal-darwin18/rbconfig.rb:215: warning: Insecure world writable dir /Users/xxxx/dev in PATH, mode 040777
Fetching: concurrent-ruby-1.1.7.gem (100%)
Successfully installed concurrent-ruby-1.1.7
Fetching: i18n-0.9.5.gem (100%)
Successfully installed i18n-0.9.5
Fetching: thread_safe-0.3.6.gem (100%)
Successfully installed thread_safe-0.3.6
Fetching: tzinfo-1.2.7.gem (100%)
Successfully installed tzinfo-1.2.7
Fetching: activesupport-4.2.11.3.gem (100%)
Successfully installed activesupport-4.2.11.3
Fetching: nap-1.1.0.gem (100%)
Successfully installed nap-1.1.0
Fetching: fuzzy_match-2.0.4.gem (100%)
Successfully installed fuzzy_match-2.0.4
Fetching: httpclient-2.8.3.gem (100%)
Successfully installed httpclient-2.8.3
Fetching: algoliasearch-1.27.3.gem (100%)
Successfully installed algoliasearch-1.27.3
Fetching: ffi-1.13.1.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing cocoapods:
	ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.3.0/gems/ffi-1.13.1/ext/ffi_c
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r ./siteconf20200810-3603-xmhpxy.rb extconf.rb
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/universal-darwin18/rbconfig.rb:215: warning: Insecure world writable dir /Users/xxxx/dev in PATH, mode 040777
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.3.0/gems/ffi-1.13.1 for inspection.
Results logged to /Library/Ruby/Gems/2.3.0/extensions/universal-darwin-18/2.3.0/ffi-1.13.1/gem_make.out