fix(postcss): fallback postcss config locations (#6119)
### Description
- closes https://github.com/vercel/turbo/issues/5068
For the postcss transform specific, there's no gaurantee if we lookup
context from the path of the source as we allow to include import
outside of the project root as resolvable. PR amends lookup behavior for
the config, that looks for the project root first then try to use source
path later.
Closes WEB-1719
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Co-authored-by: Will Binns-Smith <wbinnssmith@gmail.com>
