No thread context classloader when invoking server main, breaks ServiceLoader lookups (javax.script on Purpur)

[fzzgg]

Booting Purpur 1.21.7 with tack 0.1.1-beta (source build) dies while
loading the world. Purpur gets null back from its javax.script
lookup (Rhino), so Entity.scriptEngine is null. Same jar works
fine under plain java and under Paperclip.

Env: Debian hosts, server runs in Docker on
amazoncorretto:26-al2023 (glibc, tack needs it).

tack invokes main on a JNI-attached thread without setting a
context classloader first, so ServiceLoader falls back to
bootstrap and never sees the app classpath. Setting it before
the call, the way Paperclip does, fixes it. The snippet below
is what we run in exec_jvm (src/launcher.rs), right before
the main_class lookup, on 11 prod servers since late August:

let system_loader = env
    .call_static_method(
        jni_str!("java/lang/ClassLoader"),
        jni_str!("getSystemClassLoader"),
        jni_sig!("()Ljava/lang/ClassLoader;"),
        &[],
    )?
    .l()
    .into_error()?;
let current_thread = env
    .call_static_method(
        jni_str!("java/lang/Thread"),
        jni_str!("currentThread"),
        jni_sig!("()Ljava/lang/Thread;"),
        &[],
    )?
    .l()
    .into_error()?;
env.call_method(
    &current_thread,
    jni_str!("setContextClassLoader"),
    jni_sig!("(Ljava/lang/ClassLoader;)V"),
    &[JValue::Object(&system_loader)],
)?;