Respect Zig install prefix for native module

[emil-e]

Summary

• install the native module and version sidecar under Zig's install prefix instead of escaping it
• update Makefile, CI/release builds, docs, and helper scripts to pass --prefix . when checkout-root artifacts are expected
• keep Emacs auto-builds safe by building into a temporary prefix and renaming the module/sidecar into place

Verification

• emacs --batch -Q -L lisp --eval '(setq byte-compile-error-on-warn t)' -f batch-byte-compile lisp/ghostel-module-install.el
• emacs --batch -Q -L lisp -L test --eval '(setq load-prefer-newer t)' -l ert -l test/ghostel-test-helpers.el -l test/ghostel-module-test.el --eval "(ert-run-tests-batch-and-exit '(not (tag native)))"
• zig build --prefix /private/tmp/ghostel-pr-prefix-test -Doptimize=Debug -Dcpu=baseline
• git diff --check

[Vonfry]

With a glance, LGTM. And it is tested on linux.