Fix bug Intel Graphics Update tool for Linux* OS (the public key is not available: NO_PUBKEY 611B903CAB97EA77)

Fix bug "the public key is not available: NO_PUBKEY 611B903CAB97EA77"

$ sudo apt update
...                       
Err:7 https://download.01.org/gfx/ubuntu/17.04/main zesty InRelease   
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 611B903CAB97EA77
...                             
Reading package lists... Done                                                                                                                 
W: GPG error: https://download.01.org/gfx/ubuntu/17.04/main zesty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 611B903CAB97EA77
E: The repository 'https://download.01.org/gfx/ubuntu/17.04/main zesty InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Solution:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 611B903CAB97EA77
sudo apt-get update