Update "cross-env" to the latest version, which includes a fix for co…

…mmands in Windows with undefined env variables

Fixes #16452
Closes #19084

The latest version of `cross-env` (`5.1.1`) includes a fix to #16452.
The fix is: kentcdodds/cross-env#149
Now the behaviour of commands like `cross-env node $UNDEFINED_VAR index.js` is expanding the variable
to the empty string, both in Windows and in UNIX.

To get the new version of `cross-env` working, I had to change all `cross-env` commands to `cross-env-shell`,
as this was a breaking change on `cross-env@5`.

To test: run `npm start` on a Windows `cmd.exe` shell, check that it works.

cc/ @Viper007Bond @withinboredom