test runner: pass DMD to druntime build so it isn't rebuilt

ensureDruntime invoked the druntime Makefile without DMD=, so its default
DMD path (generated/<os>/<build>/<model>/dmd) pointed at a nonexistent
binary on the VisualD CI (dmd lives under RelWithAsserts/), triggering a
"make -C .. dmd" that failed compiling build.d against absent Phobos.
Pass the existing compiler explicitly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dkorpel

compiler/test/run.d

@@ -258,6 +258,10 @@ void ensureDruntime(const string[string] env)
         make, "-C", druntimeDir,
         "MODEL=" ~ env["MODEL"],
         "BUILD=" ~ runtimeBuild(env["BUILD"]),
+        // Pass the existing compiler so the druntime Makefile doesn't try to
+        // (re)build dmd itself, which fails when the compiler lives in a
+        // different generated/ subdirectory (e.g. VisualD's RelWithAsserts).
+        "DMD=" ~ env["DMD"],
     ];
 
     writefln("Building druntime: %-(%s %)", buildCommand);