Fix bug in gravity.sh where IPv6 list was not always populated

Failed to be populated if /tmp/piholeIP does not exist. This may happen under
the following conditions:

- was not generated during install
- gravity.sh being run again after the original install (the file is deleted
  during the installation process)

gravity.sh

@@ -13,10 +13,6 @@
 piholeIPfile=/tmp/piholeIP
 piholeIPv6file=/etc/pihole/.useIPv6
 if [[ -f $piholeIPfile ]];then
-  if [[ -f $piholeIPv6file ]];then
-    # If the file exists, then the user previously chose to use IPv6 in the automated installer
-    piholeIPv6=$(ip -6 route get 2001:4860:4860::8888 | awk -F " " '{ for(i=1;i<=NF;i++) if ($i == "src") print $(i+1) }')
-  fi
     # If the file exists, it means it was exported from the installation script and we should use that value instead of detecting it in this script
     piholeIP=$(cat $piholeIPfile)
     rm $piholeIPfile
@@ -27,6 +23,11 @@ else
     piholeIP=${piholeIPCIDR%/*}
 fi
 
+if [[ -f $piholeIPv6file ]];then
+    # If the file exists, then the user previously chose to use IPv6 in the automated installer
+    piholeIPv6=$(ip -6 route get 2001:4860:4860::8888 | awk -F " " '{ for(i=1;i<=NF;i++) if ($i == "src") print $(i+1) }')
+fi
+
 # Ad-list sources--one per line in single quotes
 # The mahakala source is commented out due to many users having issues with it blocking legitimate domains.
 # Uncomment at your own risk