Resolve ghcup isolation installation failure in Haskell autotester

[donny-wong]

Context

During the Docker container build and initialization phase, the Haskell toolchain installer (ghcup) was failing when executing /app/autotest_server/testers/haskell/requirements.system. This failure returned a non-zero exit code (100).

Cause of the Issue

The script previously passed the --isolate /usr/local/bin flag to ghcup. However, ghcup includes a strict safety guard: it will refuse to unpack assets into an isolation target directory if that directory is not completely empty. Because modern base images (like ubuntu:24.04) pre-populate /usr/local/bin with standard system binaries and symlinks, ghcup aborted with a fatal error:
[ Error ] Install directory /usr/local/bin is not empty.

Changes Implemented

1. Isolated Sandboxing: Updated requirements.system to create a dedicated, guaranteed-empty subdirectory: /usr/local/bin/stack-isolated.
2. Global Symlinking: Directed ghcup to extract stack cleanly into this empty directory, then created a system-wide symlink at /usr/local/bin/stack pointing to the isolated binary (/usr/local/bin/stack-isolated/stack). This satisfies ghcup's safety checks while keeping the binary universally accessible to the tester framework.
3. Robust Home Expansion: Swapped out the $HOME environment variable for shell tilde (~) expansion to ensure absolute home path resolution regardless of the active Docker build context.