Update BuildLinux.sh (#2919)

getting the value for FOUND_GTK* variables seems to cause an error which prevents using the -u option to install the needed packages. This is fixed by moving the "exit on first error" flag to after setting these vars.

BuildLinux.sh

@@ -1,5 +1,4 @@
 #!/bin/bash
-set -e # exit on first error
 
 export ROOT=`pwd`
 export NCORES=`nproc --all`
@@ -7,6 +6,8 @@ export CMAKE_BUILD_PARALLEL_LEVEL=${NCORES}
 FOUND_GTK2=$(dpkg -l libgtk* | grep gtk2)
 FOUND_GTK3=$(dpkg -l libgtk* | grep gtk-3)
 
+set -e # exit on first error
+
 function check_available_memory_and_disk() {
     FREE_MEM_GB=$(free -g -t | grep 'Mem:' | rev | cut -d" " -f1 | rev)
     MIN_MEM_GB=10