[RISCV][CI] Fix the kernel URL to point to snapshot archives

[moste00]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository.
• I made sure to follow the project's coding style.
• I've documented every RZ_API function and struct this PR changes.
• I've added tests that prove my changes are effective (required for changes to RZ_API).
• I've updated the Rizin book with the relevant information (if needed).
• I've used AI tools to generate fully or partially these code changes and I'm sure the changes are not copyrighted by somebody else.

Detailed description

The kernel fetch URL in rv64 VM workflow used to point to a URL like https://ftp.debian.org/debian/pool/main/l/linux/<version>.deb, this URL is essentially just a reference into a live pool of used images by current releases, when no relase references it it gets garbage collected out of existence.

The URL can be replaced by a snapshot URL which is more stable.

Test plan

The CI is green again.

Closing issues

closes #6624

Long Term

For elegance/de-duplication reason, we should perhaps use what riscv-32 uses, which is https://github.com/moste00/riscv-minstack, a self-hosted minimal build of linux, open SBI, and busybox. RV32 must do this because it has no choice, as almost no mainstream linux distro publishes pre-built images for rv32. RV64 should probably exploit this and also depend on the same build, possibly after transferring the repo to Rizinorg.

[Rot127]

Thanks