JOLT_TRACE: honor the env at runtime in a built joltc

The JOLT_TRACE opt-in was a top-level form in compile-eval.ss, so in a
self-contained joltc it ran at heap-build time — where JOLT_TRACE is always
unset — and never at runtime. `JOLT_TRACE=1 joltc -M:run` therefore produced no
trace from the distributed binary (it worked only under the source-loaded dev
launcher). REPL/nREPL tracing was unaffected (those enable at runtime).

Make it a jolt-trace-init-from-env! fn called from the runtime entrypoints — the
cli.ss dispatch and the built-joltc launcher — before any app namespace compiles,
so the app's own code is traced. While here, drop a redundant trace print in the
joltc launcher (jolt-report-throwable already emits it) that double-printed the
block once tracing actually produced one.

joltc-selfbuild-smoke asserts JOLT_TRACE=1 through the built binary yields exactly
one tail-frame trace.

host/chez/build-joltc.ss

@@ -145,11 +145,10 @@
 (scheme-start
   (lambda args
     (set-source-roots! (list \"jolt-core\" \"stdlib\"))
-    (guard (v (#t (jolt-report-throwable v (current-error-port))
-                  (let ((bt (jolt-backtrace-string v)))
-                    (when bt (display \"  trace:\\n\" (current-error-port))
-                          (display bt (current-error-port))))
-                  (exit 1)))
+    ;; JOLT_TRACE at RUNTIME (the env is unset at heap-build), before any app ns
+    ;; compiles, so a `-M:run` traces the app's own code.
+    (jolt-trace-init-from-env!)
+    (guard (v (#t (jolt-report-throwable v (current-error-port)) (exit 1)))
       (cond
         ((and (= (length args) 2) (string=? (car args) \"-e\"))
          (let ((result (jolt-final-str