[BACKPORT 2.18][PLAT-12610] Fixing scripts for cron ubuntu

Summary:
Original commit: c095598 / D37141
On ubuntu cronjobs it appears the $USER variable is not set so the scripts we execute like zip_purge_logs or clean_cores might not work. This diff adds sensible default of whoami output.

Test Plan: add patch to ubuntu cron, ensure $USER errors go away

Reviewers: svarshney, amalyshev

Reviewed By: svarshney

Subscribers: yugaware

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D37224

managed/devops/roles/configure-cluster-server/templates/clean_cores.sh.j2

@@ -41,6 +41,7 @@ while [[ $# -gt 0 ]]; do
   shift
 done
 
+USER=${USER:-$(whoami)}
 if [[ "$(id -u)" != "0" && "$USER" != "yugabyte" ]]; then
   echo "This script must be run as root or yugabyte"
   exit 1

managed/devops/roles/configure-cluster-server/templates/collect_metrics_wrapper.sh.j2

@@ -13,6 +13,7 @@ set -euo pipefail
 collect_metrics_script=({{ yb_home_dir }}/bin/collect_metrics.sh)
 filename=({{ yb_metrics_dir }}/node_metrics.prom)
 
+USER=${USER:-$(whoami)}
 if [[ "$(id -u)" != "0" && "$USER" != "yugabyte" ]]; then
   echo "This script must be run as root or yugabyte"
   exit 1

managed/devops/roles/configure-cluster-server/templates/zip_purge_yb_logs.sh.j2

@@ -63,6 +63,7 @@ while [[ $# -gt 0 ]]; do
   shift
 done
 
+USER=${USER:-$(whoami)}
 if [[ "$(id -u)" != "0" && "$USER" != "yugabyte" ]]; then
   echo "This script must be run as root or yugabyte" >&2
   exit 1