feat: add RISC-V 64-bit platform support

[gounthar]

Summary

• Add riscv64 → linuxriscv64 mapping to the test platform map in UnixUtils.groovy
• Add 4 Cucumber BDD scenarios for Java installation on RISC-V (install, default install, failed download, unsupported version)
• Include test fixtures (tar.gz archives + hook scripts) for linuxriscv64
• Fix the machine step definition to synchronize var/platform with the stubbed architecture, enabling correct WireMock API routing for non-host platforms

Motivation

RISC-V 64-bit (riscv64) hardware is increasingly available, but SDKMAN classifies it as "Exotic", preventing Java candidate listing/installation. While the full fix requires changes to the installer (get.sdkman.io) and backend (DISCO API), this PR prepares the CLI's test infrastructure so that:

1. Running tests on RISC-V hardware produces the correct linuxriscv64 platform string
2. RISC-V installation flows are verified through Cucumber scenarios
3. The platform descriptor synchronization fix also benefits any future non-x86_64 platform additions

Fixes #1321

Test plan

• All 179 tests pass (including 4 new RISC-V scenarios)
• Existing x86_64 and platform-agnostic scenarios unaffected
• ./gradlew clean test --info passes on Linux x86_64

Changes

File	Change
UnixUtils.groovy	Add "riscv64": "linuxriscv64" to Linux platform map
initialisation_steps.groovy	Sync var/platform file when machine architecture is set
java_installation.feature	Add 4 RISC-V scenarios mirroring x86_64 test coverage
jdk-8.0.{111,101}-linuxriscv64.tar.gz	Test fixture archives (copies of linuxx64 fixtures)
post_hook_java_8.0.{111,101}_linuxriscv64.sh	Hook scripts (copies of linuxx64 hooks)