RuboCop: Use absolute path for cache root directory

What does this MR do?

Use absolute path for RuboCop's cache root directory.

This avoids the creation of orphan directories when workspace root is changed. For example: VSCode Ruby plugin.

Using Dir.getwd is safe because RuboCop is using Dir.chdir right before reading the configuration files to get the proper directory context.

Both Using __dir__ (➡ .) and __FILE__ (➡ (erb)) won't work.

Refs !64321 (comment 651140854) gitlab-foss!28672 (merged) https://github.com/rubyide/vscode-ruby/issues/594

Screenshots or Screencasts (strongly suggested)

n/a

How to setup and validate locally (strongly suggested)

Before

1. cd app/models
2. bundle exec rubocop wiki.rb
3. ls -al tmp/rubocop_cache ➡ directory created 👀 ❌

After

1. cd app/models
2. bundle exec rubocop wiki.rb
3. ls -al tmp/rubocop_cache ➡ nothing created 🎉 ✔

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team