[install.sh] Fix CPU cstates configuration. (sonic-net#7222)

The motivation of these changes is to fix (sonic-net#6051):

- Why I did it
To fix CPU cstates configuration

- How I did it
Updated code to be POSIX compatible

- How to verify it
root@sonic:/home/admin# sonic_installer install sonic-mellanox.bin

Signed-off-by: Nazarii Hnydyn <nazariig@nvidia.com>

installer/x86_64/install.sh

@@ -562,9 +562,10 @@ trap_push "rm $grub_cfg || true"
 [ -r ./platform.conf ] && . ./platform.conf
 
 # Check if the CPU vendor is 'Intel' and disable c-states if True
-CPUVENDOR=$(cat /proc/cpuinfo | grep -m 1 vendor_id | awk '{print $3}')
+CPUVENDOR="$(cat /proc/cpuinfo | grep -m 1 vendor_id | awk '{print $3}')"
 echo "Switch CPU vendor is: $CPUVENDOR"
-if [[ $(echo $CPUVENDOR | grep -i "Intel") ]] ; then
+if echo "$CPUVENDOR" | grep -i 'Intel' >/dev/null 2>&1; then
+    echo "Switch CPU cstates are: disabled"
     CSTATES="intel_idle.max_cstate=0"
 else
     CSTATES=""