fix: check if system_tests args are empty

When `system_tests` is called without arguments, `[ -n "$@" ]` becomes
`[ -n ]`, which is true, resulting in running `export`, which lists all
exported variables, unnecessarily cluttering the output.
If `system_tests` is called with more than one argument, the check would
fail because having more than one argument after `-n` is invalid syntax.
Fix this by using `$*`, which concatenates all positional arguments to
`system_tests` into a single value.

Part of #12636

build_tools/check.sh

@@ -124,7 +124,7 @@ fi
 
 # Using "()" not "{}" because we do want a subshell (for the export)
 system_tests() (
-    [ -n "$@" ] && export "$@"
+    [ -n "$*" ] && export "$@"
     export FISH_GETTEXT_EXTRACTION_DIR="$gettext_template_dir"
     "$workspace_root/tests/test_driver.py" "$build_dir"
 )