fix: npm exec does not use script-shell option to run commands

[lux01]

npm exec/npx do not fully respect the script-shell parameter. When combined with npm/run-script#103 this means that users on AIX cannot run npx/npm exec commands even with script-shell set to bash.

For example:

$ npm config get script-shell
bash
$ npx

Entering npm script environment at location:
/home/gb120268/package/bin
Type 'exit' or ^D when finished

bash: --:  not found
$ npm config delete script-shell
$ ./npx

Entering npm script environment at location:
/home/gb120268/package/bin
Type 'exit' or ^D when finished

sh: --:  not found

References

Related to npm/run-script#103

[wraithgar]

Ah yep, libnpmexec is pulling scriptShell out of flatOptions, but not passing it back through the chain to run-script

[wraithgar]

libnpmexec tests need to be refactored to use spawk. That's outside the scope of this PR though.