use mamba instead of conda #655

Job	Run time
lint (flake8)	18s
lint (mypy)	4m 12s
execution-filter	6s
lint (black)	22s
Test API, headers and no-engine mode	3m 20s
Test ${{ matrix.execution }} execution, Python 3.8	0s
test-internals	0s
test-clean-install-${{ matrix.os }}	0s
Test HDK storage format, Python 3.8	0s
test-${{ matrix.os }} (engine ${{ matrix.engine }}, python ${{ matrix.python-version }}, ${{ matrix.test_task }})	0s
test-ubuntu (engine unidist ${{ matrix.unidist-backend }}, python ${{ matrix.python-version }})	0s
test-asv-benchmarks	0s
test experimental	0s
test-${{ matrix.os }}-sanity (engine ${{ matrix.execution.name }}, python ${{ matrix.python-version }})	0s
test (pyarrow, python ${{ matrix.python-version }})	0s
test-spreadsheet (engine ${{ matrix.engine }}, python ${{ matrix.python-version }})	0s
upload-coverage	22s
	8m 40s

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