RFR: 8339548: GHA: RISC-V: Use Debian snapshot archive for bootstrap

[Fei Yang]

Debian "sid" or "unstable" (https://httpredir.debian.org/debian) that we use for debootstrapping RISC-V breaks very often. Currently, the GHA linux-cross-build for RISC-V would not continue and is simply skipped when this debootstrap for "sid" repos fails. (See [JDK-8326960](https://bugs.openjdk.org/browse/JDK-8326960) for more details). This is affecting GHA linux-cross-build for RISC-V for quite some time. As a result, we failed to catch some early build issues.

But I don't think we need to catch up with the latest Debian "unstable" for our GHA verification purpose. So one way would be using some older but working Debian shapshot [1] [2] for our purpose. I find the most recent usable shapshot is https://snapshot.debian.org/archive/debian/20240228T034848Z/. We can switch back to more stable Debian repo once it graduates.

[1] https://snapshot.debian.org/
[2] https://lists.debian.org/debian-snapshot/

Testing:
- [ ] GHA

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Commit messages:
 - 8339548: GHA: RISC-V: Use Debian snapshot archive for bootstrap

Changes: https://git.openjdk.org/jdk/pull/20853/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20853&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339548
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20853.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20853/head:pull/20853

PR: https://git.openjdk.org/jdk/pull/20853