test: Export the mgmt command to be used during test

test/shell/t5.sh

@@ -1,7 +1,7 @@
 #!/bin/bash -e
 
 # should take slightly more than 35s, but fail if we take much longer)
-$timeout --kill-after=55s 50s ./mgmt run --yaml t5.yaml --converged-timeout=5 --no-watch --no-pgp --tmp-prefix &
+$timeout --kill-after=55s 50s "$MGMT" run --yaml t5.yaml --converged-timeout=5 --no-watch --no-pgp --tmp-prefix &
 pid=$!
 wait $pid	# get exit status
 exit $?