Update tox.ini

Author: Cadair

tox.ini

@@ -14,6 +14,7 @@ passenv =
     WINDIR
     MPL_UPDATE_*
 setenv =
+    COVERAGE_FILE = {toxinidir}/.coverage
     MPLBACKEND = Agg
     mpldev: PIP_EXTRA_INDEX_URL = https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
 changedir = .tmp/{envname}