Fix env-var name for parallel tox tests #435

Job	Run time
test (linting,docs, 3.12, false)	53s
test (py312-dj50-postgres-xdist-coverage, 3.12, false)	15m 14s
test (py311-dj50-postgres-xdist-coverage, 3.11, false)	13m 48s
deploy	0s
test (py311-dj42-postgres-xdist-coverage, 3.11, false)	13m 38s
test (py310-dj41-postgres-xdist-coverage, 3.10, false)	14m 41s
test (py310-dj32-postgres-xdist-coverage, 3.10, false)	14m 43s
test (py311-dj41-mysql_innodb-coverage, 3.11, false)	1m 47s
test (py310-dj42-mysql_innodb-coverage, 3.10, false)	1m 58s
test (py39-dj32-mysql_innodb-xdist-coverage, 3.9, false)	1m 54s
test (py38-dj41-sqlite-xdist-coverage, 3.8, false)	1m 35s
test (py38-dj32-sqlite-xdist-coverage, 3.8, false)	1m 37s
test (py310-djmain-sqlite-coverage, 3.10, true)	1m 41s
test (py38-dj32-mysql_myisam-coverage, 3.8, false)	1m 42s
test (py38-dj32-sqlite-pytestmin-coverage, 3.8, false)	1m 25s
test (pypy3-dj32-postgres, pypy3.9, false)	2m 39s
	1h 29m 15s

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