tests: always run mkdir in a clean environment

When env_use_destdir is used to run the test linked
against the libraries built with ASan/UBSan `mkdir`
fails with
```
==13272==ASan runtime does not come first in initial library list; you should either link runtime to your application or manually preload it with LD_PRELOAD
```

Signed-off-by: Evgeny Vereshchagin <[email protected]>

Author: evverx

libsepol/tests/Makefile

@@ -1,3 +1,4 @@
+ENV ?= env
 M4 ?= m4
 MKDIR ?= mkdir
 EXE ?= libsepol-tests
@@ -47,7 +48,7 @@ clean:
 
 
 test: $(EXE) $(policies)
-	$(MKDIR) -p policies/test-downgrade
+	$(ENV) -i $(MKDIR) -p policies/test-downgrade
 	../../checkpolicy/checkpolicy -M policies/test-cond/refpolicy-base.conf -o policies/test-downgrade/policy.hi	
 	./$(EXE)