Fixes the following warning when setting
`systemd.enableStrictShellChecks = true`:
> In /nix/store/a3mk99mgl10a4k3maxx361hdli5p2rip-unit-script-ensure-printers-start/bin/ensure-printers-start line 6:
> /nix/store/26fb46gwc5sbd045nj3dxw4zqpml359i-cups-2.4.11/bin/lpadmin -D 'virtual printer for cups-pdf instance pdf' -L '/var/spool/cups-pdf-pdf/users/${USER}' -m CUPS-PDF_opt.ppd -p pdf -v cups-pdf:/pdf -E
> ^-- SC2016 (info): Expressions don't expand in single quotes, use double quotes for that.
6bb5aa2d26