[PGPRO-5575]: Fixed installcheck in Makefile.

When running "make installcheck" again, the tests fell.
This change fixed this.

Author: MakSl

Makefile

@@ -30,11 +30,7 @@ ISOLATIONCHECKS = corner_cases
 
 check: isolationcheck
 
-installcheck: submake-isolation
-	$(MKDIR_P) isolation_output
-	$(pg_isolation_regress_installcheck) \
-	  --outputdir=isolation_output \
-	$(ISOLATIONCHECKS)
+installcheck: isolationcheck
 
 isolationcheck: | submake-isolation temp-install
 	$(MKDIR_P) isolation_output