Use externally defined npm if provided

This is useful for cross-compilation because the real npm command will also be
given parameters as target cpu, etc.
7 jobs for feature/npm
Status Job ID Name Coverage
  Tests
manual #45587
redshift manual
tests
 
  Package
created #45589
redshift allowed to fail manual
package:test:docker-test-image
created #45588
redshift allowed to fail manual
releases:test
 
  Quality
created #45591
redshift
audit_dependencies
created #45590
redshift
fmt_and_clippy
 
  Integration
created #45592
redshift allowed to fail manual
sync_g1
created #45593
redshift allowed to fail manual
sync_gtest