fix Nushell tests

because this was developed when Nushell was in 0.95.0, some tweaks had to be added

• allow to set the Nushell binary with the NU var in Makefile, e.g. make NU="nu-0.95.0" test if you have Nushell 0.95.0 installed as nu-0.95.0 in PATH
• use $nu.current-exe for nested Nushell invocation to preserve the version set in the top-level Makefile
• use --no-config-file to make sure there is no interaction with the user's config, which might not be for the correct version

help_func/execute_all_tests.nu

@@ -13,9 +13,9 @@ def main [--ssh-addr-file: path] {
         print $"\n(ansi yellow_reverse)    LAUNCHING TEST ($test_name)(ansi reset)\n"
 
         (if $ssh_addr_file == null {
-            nu $test_name
+            ^$nu.current-exe $test_name
         } else {
-            nu $test_name --ssh-addr-file=$ssh_addr_file
+            ^$nu.current-exe $test_name --ssh-addr-file=$ssh_addr_file
         }) e>| do { |e| # capture the possible error and check if it's empty or not
         let maybe_error = ($e | parse -r "(Error: .*)")
             if ($maybe_error | is-empty) {