test/system: Tweak

Using 'true' is likely going to be quicker than launching the entire
shell (ie., /bin/sh).

Note that 'toolbox run' already invokes a wrapper shell via capsh(1)
before invoking the user-specified command.  So, this was the second
instance of a shell.

https://github.com/containers/toolbox/pull/1145

test/system/104-run.bats

@@ -160,7 +160,7 @@ teardown() {
 @test "run: Run command exiting with zero code in the default container" {
   create_default_container
 
-  run $TOOLBOX run /bin/sh -c 'exit 0'
+  run $TOOLBOX run true
 
   assert_success
   assert_output ""