[BUGFIX][BUILD] When test fail in circle-ci in main, have a proper error message (#6416)

Since FUNCNAME is not defined when running outside a function,
trap does not work and display wrong error message.

Example from https://circleci.com/gh/hashicorp/consul/69506 :

```
  ⨯ FAIL
  /home/circleci/project/test/integration/connect/envoy/run-tests.sh: line 1: FUNCNAME[0]: unbound variable
  make: *** [GNUmakefile:363: test-envoy-integ] Error 1
```

This fix will avoid this error message and display the real cause.

test/integration/connect/envoy/run-tests.sh

@@ -64,7 +64,7 @@ function command_error {
   # printf '%s\n' "${BASH_LINENO[@]}"
 }
 
-trap 'command_error $? "${BASH_COMMAND}" "${LINENO}" "${FUNCNAME[0]}" "${BASH_SOURCE[0]}:${BASH_LINENO[0]}"' ERR
+trap 'command_error $? "${BASH_COMMAND}" "${LINENO}" "${FUNCNAME[0]:-main}" "${BASH_SOURCE[0]}:${BASH_LINENO[0]}"' ERR
 
 # Cleanup from any previous unclean runs.
 docker-compose down -v --remove-orphans