make: fix toxenv

Signed-off-by: Alessandro Comodi <acomodi@antmicro.com>

Makefile

diff --git a/Makefile b/Makefile
index 8df6859..99422c0 100644
--- a/Makefile
+++ b/Makefile
@@ -12,7 +12,7 @@
 REQUIREMENTS_FILE ?= requirements.txt
 ENVIRONMENT_FILE ?= environment.yml
 
-TOXPYTHON ?=
+TOXENV ?=
 
 V2X_PYTHON_SRCS = $(shell find v2x -name "*py")
 
@@ -24,7 +24,7 @@
 	$(IN_CONDA_ENV) yapf -i ${V2X_PYTHON_SRCS} setup.py
 
 test-py:
-	$(IN_CONDA_ENV) TOXENV=$(TOXPYTHON) tox
+	$(IN_CONDA_ENV) TOXENV=$(TOXENV) tox
 
 .PHONY: env build test-py