add mention of spfx envvar to set on dev env

[andrewconnell]

Category

• [x] Content fix
• [ ] New article

Related issues

• n/a

What's in this Pull Request?

add additional info on the envvar for new env config

[learn-build-service-prod[bot]]

Learn Build status updates of commit 07a118d:

:warning: Validation status: warnings

File	Status	Preview URL	Details
docs/spfx/set-up-your-development-environment.md	:warning:Warning	View	Details

docs/spfx/set-up-your-development-environment.md

• Line 117, Column 1: [Warning: duplicate-h2s - See documentation] Duplicate heading: 'H2 'Trusting the self-signed developer certificate''. Each second level heading (H2) in an article must be unique.
• Line 132, Column 1: [Warning: duplicate-h2s - See documentation] Duplicate heading: 'H2 'Trusting the self-signed developer certificate''. Each second level heading (H2) in an article must be unique.

For more details, please refer to the build report.

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[learn-build-service-prod[bot]]

Learn Build status updates of commit b037a9d:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs/spfx/set-up-your-development-environment.md	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel