fix(cli): don't mutate options export

[pi0]

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

Don't directly mutate the exported object from nuxt.config.js.

fixes #5840 and a workaround for standard-things/esm#813

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 #5865 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #5865      +/-   ##
==========================================
+ Coverage   95.72%   95.72%   +<.01%     
==========================================
  Files          82       82              
  Lines        2690     2691       +1     
  Branches      690      690              
==========================================
+ Hits         2575     2576       +1     
  Misses         97       97              
  Partials       18       18

Flag	Coverage Δ
#e2e	100% <ø> (ø)	⬆️
#fixtures	50.35% <0%> (-0.02%)	⬇️
#unit	92.71% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
packages/cli/src/utils/config.js	93.75% <100%> (+0.2%)	⬆️

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[atinux]

Nice catch 👏