tool/extlibs.rb:6:in `require': cannot load such file -- digest (LoadError)

Ubuntu 18.04.6 で Ruby head を build しようとしたら以下のエラーになり build できなかった。

/home/vagrant/workdir/ruby/tool/extlibs.rb:6:in `require': cannot load such file -- digest (LoadError)
        from /home/vagrant/workdir/ruby/tool/extlibs.rb:6:in `<top (required)>'
        from /home/vagrant/workdir/ruby/ext/psych/extconf.rb:24:in `require_relative'
        from /home/vagrant/workdir/ruby/ext/psych/extconf.rb:24:in `<top (required)>'
        from ../ruby/ext/extmk.rb:217:in `load'
        from ../ruby/ext/extmk.rb:217:in `block in extmake'
        from /home/vagrant/workdir/ruby/lib/mkmf.rb:324:in `open'
        from ../ruby/ext/extmk.rb:213:in `extmake'
        from ../ruby/ext/extmk.rb:577:in `block in <main>'
        from ../ruby/ext/extmk.rb:573:in `each'
        from ../ruby/ext/extmk.rb:573:in `<main>'
ext/configure-ext.mk:105: recipe for target 'ext/psych/exts.mk' failed
make[1]: *** [ext/psych/exts.mk] Error 1
make[1]: Leaving directory '/home/vagrant/workdir/build'
uncommon.mk:289: recipe for target 'exts.mk' failed
make: *** [exts.mk] Error 2

https://bugs.ruby-lang.org/issues/18790 を参考に sudo apt install libyaml-dev したらビルドに成功した。