Skip permission check when using stdin option

[mtsmfm]

We face the following error when we run echo '#!/usr/bin/env ruby' | bundle exec rubocop --stdin dummy.rb:

echo '#!/usr/bin/env ruby' | bundle exec rubocop --stdin dummy.rb
Inspecting 1 file
An error occurred while Lint/ScriptPermission cop was inspecting
/app/dummy.rb.
To see the complete backtrace run rubocop -d.
.

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Lint/ScriptPermission cop was inspecting
/app/dummy.rb.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
Mention the following information in the issue report:
0.51.0 (using Parser 2.4.0.0, running on ruby 2.4.2 x86_64-linux)

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

I'm fine with the proposed changes. Just two points:

1. This needs a changelog entry.
2. The build is currently failing.

[mtsmfm]

Sorry, despite I checked the TODOs, I forgot to force push 🙇

[pocke]

👍
The builds for windows are failed unfortunately. But this pull-request looks good.