correctly determine INTEL_ROOT_DEFAULT using MKLROOT env variable (#1… #2770

Job	Run time
build-and-test-cpp-aarch64 (openblas)	4m 8s
build-and-push-docker-images	1h 51m 44s
build-and-test-cpp-x86_64 (ubuntu-20.04, mkl)	6m 17s
build-and-test-cpp-x86_64 (ubuntu-20.04, dnnl)	5m 55s
build-python-wheels (ubuntu-20.04, auto64)	2h 0m 29s
build-and-test-cpp-x86_64 (macos-11, mkl)	12m 10s
check-python-style	12s
build-python-wheels (macos-11, auto64)	42m 26s
build-and-test-cpp-x86_64 (macos-11, dnnl)	10m 31s
build-python-wheels (windows-2019, auto64)	5h 28m 52s
build-python-wheels (ubuntu-20.04, aarch64)	1h 14m 15s
build-python-wheels (macos-11, arm64)	13m 5s
build-and-deploy-docs	1m 9s
test-python-wheels (ubuntu-20.04)	8m 54s
test-python-wheels (macos-11)	5m 27s
test-python-wheels (windows-2019)	6m 26s
publish-python-wheels-on-pypi	0s
	12h 32m 0s

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