prevent uninitialized value warning if user has no home

If a user does not have a home env var rakudo will currently give the warning:

    use of uninitialized value %ENV of type Any in string context  in any  at src/gen/m-ModuleLoader.nqp:363

(this is the line that the core setting is loaded)

A common situation where a user will not have a home is a script being run by a webserver and the webserver user (eg. "www", "www-data") does not have a home, in which case that warning will appear twice in the server error logs every time a perl6 script is invoked.

Contrived situation:

    $ unset HOME
    $ perl6 -e 'say 1'
    use of uninitialized value %ENV of type Any in string context  in any  at src/gen/m-ModuleLoader.nqp:363

    use of uninitialized value %ENV of type Any in string context  in any  at src/gen/m-ModuleLoader.nqp:363

    1