tidy: Improve check_shell substitution lint (#42099)

Probably we should use a proper linter like `shellcheck`, but in the
meantime make the lint slightly smarter, so that it doesn't complaint
about e.g. `$1`. This is especially a problem when adding scripts which
use `awk`, since our lint is quite stupid and doesn't understand `''`
strings.
This fixes linting, for a simple new script introduced in
https://github.com/servo/servo/pull/41775

Testing: ./mach test-tidy still passes.

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

python/tidy/tests/shell_tidy.sh

@@ -12,3 +12,9 @@ if [ -z "${some_var}" ]; then
   echo "should have used [["
 fi
 [ -z "${another_var}" ]
+# Using $@, $1 etc. shouldn't trigger.
+echo "$@"
+if [[ -n "$1" ]]; then
+  echo "parameter 1 is $1"
+fi
+echo "item1 item2 item3" | awk '{$1=$1;print}'