shorten temporary SSH_REGRESS_TMP path

Previous path was exceeding max socket length on at least one platform (OSX)
diff --git a/regress/test-exec.sh b/regress/test-exec.sh
index 6995953..40d46e3 100644
--- a/regress/test-exec.sh
+++ b/regress/test-exec.sh
@@ -327,7 +327,7 @@
 
 make_tmpdir ()
 {
-	SSH_REGRESS_TMP="$($OBJ/mkdtemp openssh-regress-XXXXXXXXXXXX)" || \
+	SSH_REGRESS_TMP="$($OBJ/mkdtemp openssh-XXXXXXXX)" || \
 	    fatal "failed to create temporary directory"
 }