Add note to use bash shell on Windows

[dawei-wang]

Fix #21873

Why:

Closes #21873

What's being changed (if available, include any code snippets, screenshots, or gifs):

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Content directory changes

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Source	Preview	Production	What Changed
actions/security-guides/encrypted-secrets.md	fpt ghec ghes@ 3.8 3.7 3.6 3.5 3.4 ghae	fpt ghec ghes@ 3.8 3.7 3.6 3.5 3.4 ghae

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[cmwilson21]

@dawei-wang Thanks so much for opening a PR! I'll get this triaged for review ⚡

[skedwards88]

Thanks for this fix! We'll get this merged down for you.

[github-actions]

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