Standalone: Fix npm command handling issue

[medikoo]

Issue observed in CI fails, after refactoring plugin install command.

It appeared npm commands were not configured correctly for spawn in the context of standalone installations.

Additionally upgraded to rely on npm that recognizes node.js v14 (as standalone build now is based on Node.js v14)

[codecov]

Codecov Report

Merging #10014 (cb3b0db) into master (cd9b823) will not change coverage.
The diff coverage is 77.77%.

@@           Coverage Diff           @@
##           master   #10014   +/-   ##
=======================================
  Coverage   86.03%   86.03%           
=======================================
  Files         332      332           
  Lines       13248    13248           
=======================================
  Hits        11398    11398           
  Misses       1850     1850           

Impacted Files	Coverage Δ
scripts/pkg/build.js	0.00% <0.00%> (ø)
commands/plugin-install.js	63.23% <66.66%> (ø)
lib/cli/interactive-setup/service.js	100.00% <100.00%> (ø)
lib/plugins/plugin/install.js	94.31% <100.00%> (ø)
lib/utils/npm-command-deferred.js	75.00% <100.00%> (ø)

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