Modify download-pip-cache to not use deprecated pip options

[kragniz]

Looks like we're using some deprecated pip options:

[nix-shell:~/git/dewi]$ ./download-pip-cache 
+ download_pip ./boatd
+ dir=./boatd
+ mkdir -p pip-cache
+ pip install --download pip-cache --no-use-wheel ./boatd
DEPRECATION: --no-use-wheel is deprecated and will be removed in the future.  Please use --no-binary :all: instead.
DEPRECATION: pip install --download has been deprecated and will be removed in the future. Pip now has a download command that should be used instead.