Re-enable downstream builds for spl and openbook-dex

[KirillLykov]

Problem

We disabled downstream project builds in this commit

Why: due to upgrade of borsh from 0.9.3 to 0.10.3

Proposed Solution

To re-enable them "we'll have to add a fix to maintain borsh's transitive dependency on hashbrown to 0.12.3, since cargo will automatically upgrade it during the downstream build, causing the program builds to fail." (#30975 (comment))

• Re-enable spl Re-enable spl downstream build #32429
• Re-enable openbook-dex

[KirillLykov]

One way to resolve spl problem is to add getrandom dependency: solana-labs/solana-program-library#4703

[KirillLykov]

Looks like spl build has been disabled again. But now I believe it can be re-enabled.
My understanding is that these scripts we used before for buildkite are deprecated because we migrated to gitactions

Regarding re-enabling openbook, we need to add openbook-dex-v2 which is a new version written from scratch. I will take it but I might need some guidance from @yihau regarding these gitactions

[yihau]

sure. in #32677, I added some new files

1. .github/scripts/purge-ubuntu-runner.sh => a tool to get more space on a runner. if you don't use all default space, you don't need it.
2. .github/scripts/downstream-project-spl-common.sh => I extracted logic from old scripts. you can do similar thing to openbook-dex or just write it in an action
3. .github/workflows/downstream-project-spl.yml => the main script we need. I added workflow_call to have it reusable
4. .github/workflows/downstream-project-spl-nightly.yml => basically run .github/workflows/downstream-project-spl.yml but triggering by the cron mechanism.