Improve "running as root" check

Small issue with the USER environment variable:

  $ echo $USER
  thib
  $ sudo sh -c 'echo $USER'
  thib
  $ sudo -i sh -c 'echo $USER'
  root

Rather than recommending users to use sudo --login / -i, use the (very
widespread/portable) id program to retrieve the effective user ID
instead and don't change the recommendation.

  $ id -u
  1000
  $ sudo id -u
  0
  $ sudo -i id -u
  0

spectre-meltdown-checker.sh

@@ -256,7 +256,7 @@ fi
 
 
 /bin/echo
-if [ "$USER" != root ]; then
+if [ "$(id -u)" -ne 0 ]; then
 	/bin/echo "Note that you should launch this script with root privileges to get accurate information"
 	/bin/echo "You can try the following command: sudo $0"
 fi