fix(cli): server should not be initialized in nuxt build

[clarkdo]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

As server.js and vue-renderer are also be created in building, so server.ready and loaderResources will be executed which should only be executed in dev or start, this pr is for avoiding calling server ready and incorrect options change (like modern mode detect) which is only for server rendering.

@pi0 I removed _build in dev since it should be only for build. And I didn't see any where is using it, if there is any concern, I can add another filed in build.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #6441 into dev will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##             dev   #6441   +/-   ##
=====================================
  Coverage   95.6%   95.6%           
=====================================
  Files         79      79           
  Lines       2684    2684           
  Branches     692     692           
=====================================
  Hits        2566    2566           
  Misses       101     101           
  Partials      17      17

Flag	Coverage Δ
#e2e	100% <ø> (ø)	⬆️
#fixtures	50.7% <0%> (ø)	⬆️
#unit	92.28% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
packages/cli/src/commands/build.js	86.36% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 75f8564...7db98a7. Read the comment docs.