Download and verify GPG keys in update.sh
Download and verify GPG keys in update.sh

This also switches from using the user's personal keyring to using a
temporary keyring for `update.sh` and fingerprints hardcoded in
`update.sh`.

matt2005/raspbian-ua-netinst,malsony/raspbian-ua-netinst,FooDeas/raspberrypi-ua-netinst,racing-tech/rrp-raspi-installer,beylsp/rpi-installer,kolargol/raspbian-ua-netinst,bbinet/raspbian-ua-netinst,beylsp/rpi-installer,cybercircuits/raspbian-ua-netinst,racing-tech/rrp-raspi-installer,Zeerix/raspbian-ua-netinst,jturner314/raspbian-ua-netinst,firepick/FireHawk,dicer/raspbian-ua-netinst,yodaaut/raspbian-ua-netinst,daytonpid/FireHawk