fwget: Fix quoting

This kind of quoting doesn't work, no idea why shellcheck wanted this.
If there is a "safer" way to "fix" this feel free to do it just test
that it's working after.

Reported by: Kenneth Raplee <kenrap@kennethraplee.com>
Sponsored by: Beckhoff Automation GmbH & Co. KG
Fixes: 7ad4d94d5bf2 ("Fix some shell issues by adding quotes and replace backticks with $()")