fix(cli): manual config lookup to handle gitignored folders, fixes #3527

[FabianLars]

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• Bugfix
• Feature
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• Code style update
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Does this PR introduce a breaking change?

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Other information

Turns out more people than expected have (parts of) their project gitignored and we currently only check if $CWD/tauri.conf.json exists which doesn't work for the most common structure == the one you get via CTA/tauri init.

The proposed workaround is really straightforward, we just manually check for src-tauri/tauri.conf.json before starting the actual lookup.
A nice-to-have side-effect is that this slightly improves performance for projects with the default structure 🤷