[riscv-port-jdk11u:riscv-port] RFR: 8328948: GHA: Restoring sysroot from cache skips the build after JDK-8326960

[Fei Yang]

Fixes a recent GHA regression. Applies cleanly.

- [ ] GHA, first run passes cross-builds
- [ ] GHA, second run passes cross-builds

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Commit messages:
 - backport

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/15/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=15&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328948
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/15.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/15/head:pull/15

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/15