fatal: 'submodule' appears to be a git command, but we were not able to execute it. Maybe git-submodule is broken?

[JimiC]

OS: Windows 10
Node: Any
npm: 6.1.0

As mentioned in npm/npm#20815

[JimiC]

The commit to blame is e4ecc54

Using https://github.com/laggingreflex/npm-lifecycle-windows-path-issue with node@10.4.1 and npm@6.1.0 with the offending code stripped, npm test works as expected.

[JimiC]

Looking at index.js code I cannot justify the addition of

if (process.platform === 'win32') {
    env.path = env.PATH = env.Path = env[PATH]
 }

At line https://github.com/npm/npm-lifecycle/blob/v2.0.3/index.js#L24 the code does the handling for the case that the Path environment variable in Windows is defined with different casing.
Now even in the case where Path is not defined at all (which is highly unlikely), line 25 makes sure that it gets defined using the name Path. Therefore

npm-lifecycle/index.js

Line 124 in e4ecc54

env[PATH] = pathArr.join(process.platform === 'win32' ? ';' : ':')

sets the variable with whatever casing it was found and in case it was not it's getting defined as Path.

I really cannot think why @laggingreflex code did not work for him at first. Maybe the node version he used was buggy causing the issue.