Relates to npm/cli#2921

This issue arises after moving from npm 6.x to 7.x or later. As the
build agents are still on 6.x, we have not yet seen this issue there.

As part of the release build for Keyman Developer, we bundle up some
files and do some work in a temp folder to prepare folders for release.
Even though we are only working with `dependencies` (e.g. using `npm
install --production`), packages under `devDependencies` are still
wrongly being checked by NPM. As we have two modules present in
`devDependencies` that are only available when in the repo path, we need
to remove them before attempting to continue. Yuck!

ref: npm/cli#3975 (comment)
ref: npm/cli#2921

This made the string processing needed much simpler than
when it was embedded within the Makefile, where horrible
escaping proved necessary.