tests: Fix test_scikit_ci ensuring variable SCIKIT_CI_<STEP> are cleared
tests: Fix test_scikit_ci ensuring variable SCIKIT_CI_<STEP> are cleared

This commit fixes a regression introduced in 7d6de1f (ci: Implement step
ordering and automatic dependent step execution. Fixes #9)

It turns that using "lower()" fixed the appveyor issues that were
reported while on working on the previous PR, indeed windows environment
variable are case insensitive [1]

That said, using the correct case is now needed to fix testing failure
occurring after the previous commit 8e5d9af (ci: Simplify
appveyor/circle/travis configuration)

[1] https://en.wikipedia.org/wiki/Environment_variable#Windows_2

scikit-build/scikit-ci