[now-next] Invoke build script

[thasophearak]

This PR make @now/next invoke build script if user defined.

PRODUCT-106 #close

[codecov-io]

Codecov Report

Merging #3073 into canary will increase coverage by 2.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           canary    #3073      +/-   ##
==========================================
+ Coverage   11.33%   13.45%   +2.11%     
==========================================
  Files         268      268              
  Lines       10064    10319     +255     
  Branches     1314     1215      -99     
==========================================
+ Hits         1141     1388     +247     
- Misses       8830     8869      +39     
+ Partials       93       62      -31

Impacted Files	Coverage Δ
src/util/metrics.ts	68.18% <0%> (-2.41%)	⬇️
src/util/deploy/process-deployment.ts	0% <0%> (ø)	⬆️
src/util/dev/builder-cache.ts	80.74% <0%> (+3.47%)	⬆️
src/util/projects/get-project-by-id-or-name.ts	28.57% <0%> (+3.57%)	⬆️
src/util/get-files.ts	91.66% <0%> (+3.88%)	⬆️
src/util/dev/router.ts	82.69% <0%> (+4.12%)	⬆️
src/util/dev/builder.ts	76.92% <0%> (+4.4%)	⬆️
src/util/dev/yarn-installer.ts	90.74% <0%> (+5.02%)	⬆️
src/util/dev/server.ts	66.43% <0%> (+5.41%)	⬆️
src/util/prefer-v2-deployment.ts	61.9% <0%> (+5.65%)	⬆️
... and 1 more

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[timneutkens]

This seems like a pretty big breaking change as you're preferring build over now-build. It seems like it should prefer now-build and then try and check for build if it doesn't exist.

[timneutkens]

Great work @Sophearak 🙏