fix show techsupport date issue (sonic-net#3437)

What I did
Show techsupport is designed to collect logs and core files since given date.
I find that some core files are missing when given date is relative, for example "5 minutes ago".
Microsoft ADO: 28737486

How I did it
Create the reference file at the start of the script, and don't update it in find_files.

How to verify it
Run end to end test: show_techsupport/test_auto_techsupport.py

scripts/generate_dump

@@ -1120,7 +1120,6 @@ save_file() {
 find_files() {
     trap 'handle_error $? $LINENO' ERR
     local -r directory=$1
-    $TOUCH --date="${SINCE_DATE}" "${REFERENCE_FILE}"
     local -r find_command="find -L $directory -type f -newer ${REFERENCE_FILE}"
 
     echo $($find_command)
@@ -1914,6 +1913,8 @@ main() {
     ${CMD_PREFIX}renice +5 -p $$ >> /dev/null
     ${CMD_PREFIX}ionice -c 2 -n 5 -p $$ >> /dev/null
 
+    # Created file as a reference to compare modification time
+    $TOUCH --date="${SINCE_DATE}" "${REFERENCE_FILE}"
     $MKDIR $V -p $TARDIR
 
     # Start with this script so its obvious what code is responsible