Make current matrix work nested under nvcc field

[jrhemstad]

As part of #239, we want to nest the current matrix environment definitions under a nvcc: field and make the workflow work.

We'll primarily need to update the compute-matrix reusable workflow to reference the nvcc: field. The outputs of this workflow also presuppose an nvcc job, so we'll need to figure out how to generalize the outputs:

cccl/.github/workflows/compute-matrix.yml

Lines 14 to 26 in ce6a462

	outputs:
	FULL_MATRIX:
	description: "The full matrix of build configurations for the given matrix type."
	value: ${{ jobs.compute-matrix.outputs.FULL_MATRIX }}
	CUDA_VERSIONS:
	description: "The unique CUDA versions in the full matrix."
	value: ${{ jobs.compute-matrix.outputs.CUDA_VERSIONS }}
	COMPILERS:
	description: "The unique compilers in the full matrix."
	value: ${{ jobs.compute-matrix.outputs.COMPILERS }}
	PER_CUDA_COMPILER_MATRIX:
	description: "The matrix of build configurations for each CUDA version and compiler."
	value: ${{ jobs.compute-matrix.outputs.PER_CUDA_COMPILER_MATRIX }}