Issue #2139: switch tarball install flow to CloudFront indexes

[rwilliamspbg-ops]

Summary\nSwitch release-tarball lookup/download in install_rocm_from_artifacts.py from direct S3 bucket API calls to CloudFront tarball indexes.\n\n## Why\nIssue #2139 tracks that prerelease tarballs are CloudFront-only and should not depend on public S3 bucket access.\n\n## Changes\n- Parse tarball asset names from CloudFront index content\n- Support both anchor href listings and JavaScript "name" file-list format\n- Resolve latest nightly release from CloudFront index\n- Download release tarballs from CloudFront URLs\n- Update release-version help text to CloudFront endpoints\n- Update tarball table in RELEASES.md to prefer CloudFront indexes\n\n## Validation\n- python build_tools/install_rocm_from_artifacts.py --latest-release --amdgpu-family gfx110X-all --dry-run\n- python build_tools/install_rocm_from_artifacts.py --release 6.4.0.dev0+8f6cdfc0d95845f4ca5a46de59d58894972a29a9 --amdgpu-family gfx120X-all --dry-run\n\nCloses #2139

[ScottTodd]

Please format the PR description

[rwilliamspbg-ops]

Issue #2139 tracks the move away from direct S3 bucket access for release tarballs. Prerelease tarballs are CloudFront-only, so the installer needs to use CloudFront tarball indexes instead of S3 API calls.

Technical Details
Switched release tarball lookup and download in install_rocm_from_artifacts.py from direct S3 API calls to CloudFront tarball indexes.
Added parsing for tarball asset names from the CloudFront index content.
Supported both anchor-style listings and the JSON file-list format used by the tarball index.
Updated latest-nightly release discovery to read from the CloudFront index.
Updated release download logic to fetch tarballs from CloudFront URLs.
Updated RELEASES.md to point users at the CloudFront tarball indexes instead of the old S3 bucket links.
Test Plan
python install_rocm_from_artifacts.py --latest-release --amdgpu-family gfx110X-all --dry-run
python install_rocm_from_artifacts.py --release 6.4.0.dev0+8f6cdfc0d95845f4ca5a46de59d58894972a29a9 --amdgpu-family gfx120X-all --dry-run
Test Result
Latest nightly dry-run resolved the expected gfx110X-all tarball.
Dev-release dry-run resolved the expected gfx120X-all tarball.
Submission Checklist
Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
Closes #2139