Change nextjs detection in vc build

[jaredpalmer]

Instead of trusting framework settings in .vercel/project.json, use the presence of .next directory after the build command is run

[codecov]

Codecov Report

Merging #7020 (adf9aa0) into main (7d7f3df) will not change coverage.
The diff coverage is n/a.

❗ Current head adf9aa0 differs from pull request most recent head 986b460. Consider uploading reports for the commit 986b460 to get more accurate results

@@           Coverage Diff           @@
##             main    #7020   +/-   ##
=======================================
  Coverage   51.49%   51.49%           
=======================================
  Files         128      128           
  Lines        4979     4979           
  Branches     1172     1172           
=======================================
  Hits         2564     2564           
  Misses       2407     2407           
  Partials        8        8           

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