druntime/test/shared loadDR: Pass -rpath on musl

The test fails on musl without the flag:
```
/usr/bin/cc  -g -Wall       src/loadDR.c  -ldl  -o /root/build/runtime/druntime-test-shared-debug/loadDR
Testing loadDR
/root/build/runtime/druntime-test-shared-debug/loadDR /root/build/lib/libdruntime-ldc-debug-shared.so.111.0
Aborting from rt/sections_elf_shared.d(966) Failed to get library handle.gmake: *** [../common.mak:192: /root/build/runtime/druntime-test-shared-debug/loadDR.done] Aborted (core dumped)
```

Signed-off-by: Andrei Horodniceanu <a.horodniceanu@proton.me>

Author: the-horo

runtime/druntime/test/shared/Makefile

@@ -152,6 +152,9 @@ $(ROOT)/loadDR.done: $(ROOT)/lib$(DOTDLL) $(DRUNTIMESO)
 $(ROOT)/loadDR.done: private run_args = $(DRUNTIMESO)
 $(ROOT)/loadDR$(DOTEXE): utils.h
 $(ROOT)/loadDR$(DOTEXE): private extra_ldlibs += $(ldl)
+ifeq ($(IS_MUSL),1)
+$(ROOT)/loadDR$(DOTEXE): private extra_ldflags += -Wl,-rpath,$(druntimeso_dir)
+endif
 
 $(ROOT)/host.done: $(DRUNTIMESO) $(ROOT)/plugin1$(DOTDLL) $(ROOT)/plugin2$(DOTDLL)
 $(ROOT)/host.done: private run_args = $(DRUNTIMESO)