Use Python instead of curl to download test file on validate-runtime-image target

[caponetto]

What changes were proposed in this pull request?

Fixes https://github.com/elyra-ai/elyra/actions/runs/12429390840/job/34702792058

Apparently, the new pytorch image doesn't include curl anymore, which causes validate-runtime-image to fail.

How was this pull request tested?

Tested on my fork through this run.

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[caponetto]

cc @lresende

[shalberd]

looks good to me, thank you, @caponetto, I have noticed the error in my PR as well.

[caponetto]

Elyra Tests / Lint Server is also failing on main so I think this PR is good to be merged.

[shalberd]

@caponetto I made some changes lint-server section related in my PR. E226 related

a8722fa#diff-4f0535277f973d16f4972b236075db911a09293effd07706b45fe787c133e18e

wondering if there, the step validate-images will run through now.