LibJSGCVerifier: Use more general shebang line and clarify instructions

My system's python3 is not in /bin/.

The README did not indicate that a clang-toolchain build of Serenity is
required, so this patch adds that explicit instruction.

Meta/Lagom/Tools/LibJSGCVerifier/README.md

@@ -7,9 +7,18 @@ two things:
 - For all types not wrapped by `GCPtr` or `NonnullGCPtr`, that the wrapped type does not inherit from `Cell`
   (otherwise it should be wrapped).
 
-Usage:
+This tool currently requires having first built Serenity with the Clang toolchain for x86_64:
+```bash
+./Meta/serenity.sh build x86_64 Clang
 ```
+
+Once Serenity is built, this tool can be built with:
+```bash
 cmake -GNinja -B build
 cmake --build build
+```
+
+Then run the tool with:
+```bash
 src/main.py -b <path to serenity>/Build
 ```

Meta/Lagom/Tools/LibJSGCVerifier/src/main.py

@@ -1,4 +1,4 @@
-#!/bin/python3
+#!/usr/bin/env python3
 
 import argparse
 import multiprocessing