Export RUST_BACKTRACE when it is not set explicitly

Currently we are overwriting the RUST_BACKTRACE if set
explicitly by user while running performance metrics tests
using dev_cli.sh

This change will allow user to set the RUST_BACKTRACE
while running performance metrics tests with dev_cli.sh
which invokes run_metrics.sh to run the performance binary.
We will set RUST_BACKTRACE to 1 if not set explicitly.

Signed-off-by: smit-gardhariya <gardhariya.smit@gmail.com>

scripts/dev_cli.sh

@@ -539,6 +539,7 @@ cmd_tests() {
             --volume "$CLH_INTEGRATION_WORKLOADS:$CTR_CLH_INTEGRATION_WORKLOADS" \
             --env USER="root" \
             --env CH_LIBC="${libc}" \
+            --env RUST_BACKTRACE="${RUST_BACKTRACE}" \
             "$CTR_IMAGE" \
             ./scripts/run_metrics.sh "$@" || fix_dir_perms $? || exit $?
     fi