Workaround for `cannot load such file -- parser (LoadError)` in CI
Workaround for `cannot load such file -- parser (LoadError)` in CI

Follow up of https://github.com/rubocop-hq/rubocop/pull/7603#issuecomment-569506218.

`Check requiring libraries successfully` of ruby-2.7-rubocop matrix
has been failed because of `require': cannot load such file -- parser (LoadError)`.

```console
$ #!/bin/bash -eo pipefail
ruby -I lib -r rubocop -e 'exit 0'
Traceback (most recent call last):
    4: from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
    3: from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
    2: from /home/circleci/project/lib/rubocop.rb:3:in `<top (required)>'
    1: from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
/usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in
`require': cannot load such file -- parser (LoadError)

Exited with code exit status 1
```

https://circleci.com/gh/rubocop-hq/rubocop/79263

jmks/rubocop,jmks/rubocop,tdeo/rubocop,bbatsov/rubocop,rrosenblum/rubocop,tejasbubane/rubocop,maxjacobson/rubocop,tejasbubane/rubocop,rrosenblum/rubocop,bbatsov/rubocop,rrosenblum/rubocop,maxjacobson/rubocop,jmks/rubocop,tdeo/rubocop,tdeo/rubocop,deivid-rodriguez/rubocop,tejasbubane/rubocop,maxjacobson/rubocop,deivid-rodriguez/rubocop,deivid-rodriguez/rubocop