Not mutate original PostCSS config file.

[hydrognomik]

↪️ Pull Request

Related to #6094.

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

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[mischnic]

Can you add a test similar to this one?

parcel/packages/core/integration-tests/test/postcss.js

Line 20 in 1ec530f

it('should support transforming css modules with postcss (require)', async () => {

Also, please revert the change to the lockfile.

[hydrognomik]

Done.

[mischnic]

Thanks!