fix(cypress/react): add default webpack config path

[lmiller1990]

• Closes @cypress/react regression in 5.9.0 from 5.8.0: findReactScriptsWebpackConfig returns undefined #16751

User facing changelog

#16751 added a config object to modify the webpack config path. We need a default for users who are not providing the configuration object.

Additional details

This is very awkward to test. We'd need another entire project - should look into this, but this fix is pretty important to get out, so I just tested it by creating a project, reproducing the error, then updating node_modules.

How has the user experience changed?

PR Tasks

• Have tests been added/updated?
• Has the original issue or this PR been tagged with a release in ZenHub?
• Has a PR for user-facing changes been opened in cypress-documentation?
• Have API changes been updated in the type definitions?
• Have new configuration options been added to the cypress.schema.json?

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[elevatebart]

Yeah, I would say that's a woops

[cypress]

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Test summary

17968 • 0 • 214 • 7 • 0

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Run details

Project	cypress
Status	Passed
Commit	53d4657
Started	Jun 4, 2021 4:05 PM
Ended	Jun 4, 2021 4:17 PM
Duration	11:54 💡
OS	Linux Debian - 10.8
Browser	Multiple

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[elevatebart]

With an example test? Even better !