[CI] Rename cuda dimension to platform in anyscale CUDA build steps

[andrew-anyscale]

Rename the cuda matrix dimension to platform (with cu-prefixed values)
in the three anyscale CUDA build steps and their wandas. The wandas
now accept $PLATFORM directly instead of constructing it from
cu$CUDA_VERSION; output is identical since callers pass cu-prefixed
values.

This aligns the dimension name with the publish steps (anyscalebuild,
anyscalellmbuild, anyscalemlbuild) which already use a platform
dimension, enabling precise ($) dependency matching on both platform
and python when these steps are later converted to array syntax.

Topic: wanda-platform-env
Relative: array-wheel-build
Signed-off-by: andrew [email protected]

[andrew-anyscale]

Reviews in this chain:
└#62201 [CI] Convert _images.rayci.yml from matrix to array syntax
　└#62202 [CI] Convert _wheel-build.rayci.yml from matrix to array syntax
　　└#62274 [CI] Rename cuda dimension to platform in anyscale CUDA build steps
　　　└#62305 [CI] Add build_short_platform_map and array key helpers
　　　　└#62273 [CI] Convert release/build.rayci.yml to array + update get_prerequisite_step
　　　　　└#62292 [CI] Add filter_release_build_yaml to trim build steps by test needs
　　　　　　└#62293 [CI] Wire up build.rayci.yml filtering in release init script

[andrew-anyscale]

#	head	base	diff	date	summary
0	35916c41	7268887e	diff	Apr 1 11:35 AM	4 files changed, 23 insertions(+), 23 deletions(-)
1	c9276986	aa202ecb	rebase	Apr 2 7:18 AM	0 files changed
2	0a16bfc2	8fd435a0	rebase	Apr 2 8:11 AM	0 files changed
3	483f20da	b1e601c7	rebase	Apr 2 8:18 AM	0 files changed
4	794e7406	290fda6a	rebase	Apr 2 8:23 AM	0 files changed
5	a0d4a619	98f547b4	rebase	Apr 2 8:28 AM	0 files changed
6	b1d1bf41	b80e07e4	rebase	Apr 2 8:33 AM	0 files changed
7	3e39fed9	eacf845f	diff	Apr 2 12:51 PM	0 files changed
8	6c31f0e2	14dfb8f6	rebase	Apr 2 12:52 PM	0 files changed
9	ce67dacd	f966b57e	rebase	Apr 2 19:33 PM	0 files changed

[gemini-code-assist]

Code Review

This pull request refactors the Buildkite and Docker configurations by replacing the cuda matrix dimension with a platform dimension, ensuring that CUDA versions are explicitly prefixed (e.g., cu12.3.2). These changes are applied across several build steps, including ray-anyscale-cuda-build, ray-llm-anyscale-cuda-build, and ray-ml-anyscale-cuda-build. Feedback indicates that for full consistency and to meet the PR's stated goals, the anyscalemlbuild step should also be updated to include the platform dimension.