RFR: 8326960: GHA: RISC-V sysroot cannot be debootstrapped due to ongoing Debian t64 transition

[Aleksey Shipilev]

Debian "unstable" is currently undergoing t64 (hello, Y2038) transition, which breaks "sid" repo that we use for debootstrapping RISC-V very often. This is seen across multiple repositories, and requires everyone to look through GHA failures all the time. This PR tries to fix this: if debootstrap for "sid" repos fails, the build for that arch would not continue. IMO, it is a better compromise for current situation. We will flip back to expecting the debootstrap to complete well after either t64 transition finishes, or we switch to more stable Debian repo once it graduates.

Additional testing:
 - [x] GHA

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/18435/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18435&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326960
  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18435.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18435/head:pull/18435

PR: https://git.openjdk.org/jdk/pull/18435