fix(eslint-plugin-tslint): fix tslintConfig memoization key

[pelepelin]

Fixes #1692

[typescript-eslint]

Thanks for the PR, @pelepelin!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

---

🙏 Please, if you or your company is finding typescript-eslint valuable, help us sustain the project by sponsoring it transparently on https://opencollective.com/typescript-eslint. As a thank you, your profile/company logo will be added to our main README which receives thousands of unique visitors per day.

[codecov]

Codecov Report

Merging #1719 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1719   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files         148      148           
  Lines        6967     6968    +1     
  Branches     2007     2009    +2     
=======================================
+ Hits         6632     6633    +1     
  Misses        124      124           
  Partials      211      211           

Flag	Coverage Δ
#unittest	95.19% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
packages/eslint-plugin-tslint/src/rules/config.ts	100.00% <100.00%> (ø)

[JamesHenry]

Thanks!