testsuite: Don't override LD_LIBRARY_PATH, only prepend

NixOS development environments often require that LD_LIBRARY_PATH
be set in order to find system libraries. T1407 was overriding
LD_LIBRARY_PATH, dropping these directories. Now it merely prepends,
its directory.

(cherry picked from commit cf4306f1)

testsuite/tests/ghci/linking/dyn/Makefile

@@ -94,7 +94,7 @@ compile_libAS_impl_msvc:
 
 .PHONY: T1407
 T1407:
-	cat T1407.script | LD_LIBRARY_PATH=. "$(TEST_HC)" $(TEST_HC_OPTS_INTERACTIVE) -L"T1407dir"
+	cat T1407.script | LD_LIBRARY_PATH=".:$(LD_LIBRARY_PATH)" "$(TEST_HC)" $(TEST_HC_OPTS_INTERACTIVE) -L"T1407dir"
 
 .PHONY: T3242
 	echo ":q" | '$(TEST_HC)' $(TEST_HC_OPTS_INTERACTIVE) -lm