Fix benchmark runner bug introduced in release 0.652 (#1530)

### Problem

In release 0.652, `RequireResolver` was refactored to add support for
`luau-analyze`.

As part of this update, `RuntimeRequireContext` introduced a new
convention where a file's chunkname must be prefixed with `@` (e.g.,
`@./some/path.luau`). This change applies to all chunknames generated
within `RuntimeRequireContext`. However, when a `.luau` file is executed
directly from the command line (e.g., `luau ./my/script.luau`), the
chunkname is still generated with the old `=` prefix (e.g.,
`=./some/path.luau`).

Since `RuntimeRequireContext` no longer recognizes chunknames prefixed
with `=`, any attempt to directly execute a `.luau` file from the
command line fails. For example, running `luau ./my/script.luau` results
in an error stating that the context is unsupported. [This issue also
affects tools like the benchmark
runner](#1525 (comment)),
which rely on direct file execution.

### Solution

Update `runFile` to replace the `=` prefix in generated chunknames with
`@`.

CLI/Repl.cpp

@@ -712,7 +712,7 @@ static bool runFile(const char* name, lua_State* GL, bool repl)
     // new thread needs to have the globals sandboxed
     luaL_sandboxthread(L);
 
-    std::string chunkname = "=" + std::string(name);
+    std::string chunkname = "@" + std::string(name);
 
     std::string bytecode = Luau::compile(*source, copts());
     int status = 0;