Refer to cheat sheet on securely storing keys rather than using environment variables

[Andrewp2]

Thank you for submitting a Pull Request (PR) to the Cheat Sheet Series.

:triangular_flag_on_post: If your PR is related to grammar/typo mistakes, please double-check the file for other mistakes in order to fix all the issues in the current cheat sheet.

Please make sure that for your contribution:

• [X] In case of a new Cheat Sheet, you have used the Cheat Sheet template.
• [X] All the markdown files do not raise any validation policy violation, see the policy.
• [X] All the markdown files follow these format rules.
• [X] All your assets are stored in the assets folder.
• [X] All the images used are in the PNG format.
• [X] Any references to websites have been formatted as [TEXT](URL)
• [X] You verified/tested the effectiveness of your contribution (e.g., the defensive code proposed is really an effective remediation? Please verify it works!).
• [x] The CI build of your PR pass, see the build status here.

If your PR is related to an issue, please finish your PR text with the following line:

This PR covers issue #1439.

Thank you again for your contribution :smiley: