Jenkins: Work around autotools/CMake adding /usr with prefix=/

This is used by e.g. LLVM jobs to install directly to tarball/ without
any additional subdirectories, but the latest refactoring broke this.

pycheribuild/jenkins.py

@@ -271,8 +271,14 @@ def _jenkins_main() -> None:
             # noinspection PyProtectedMember
             project = target._get_or_create_project_no_setup(None, cheri_config, caller=None)
             if isinstance(project, Project):
-                project._install_prefix = cheri_config.installation_prefix
-                project.destdir = cheri_config.output_root
+                # Using "/" as the install prefix results inconsistently prefixing some paths with '/usr/'.
+                # To avoid this, just use the full install path as the prefix.
+                if cheri_config.installation_prefix == Path("/"):
+                    project._install_prefix = expected_install_path
+                    project.destdir = None
+                else:
+                    project._install_prefix = cheri_config.installation_prefix
+                    project.destdir = cheri_config.output_root
                 assert project.real_install_root_dir == expected_install_path
         for tgt in cheri_config.targets:
             build_target(cheri_config, target_manager.get_target_raw(tgt))