Fix Vivado missing IP repository warning on clean builds

[taraxacum45e9a]

config.ip_repository defaults to a subdirectory inside build_dir.

SLASH/linker/slashkit/core/command_config.py

Lines 101 to 102 in a11e728

	def ip_repository(self) -> Path:
	return self.build_dir / "iprepo"

Because build_dir is wiped and recreated on every build, ip_repository may not exist yet when create_build_project is called.

SLASH/linker/slashkit/core/command_config.py

Lines 374 to 376 in a11e728

	if self._build_dir.is_dir():
	shutil.rmtree(self._build_dir)
	self._build_dir.mkdir(parents=True)

Previously it was unconditionally appended to the Vivado command line, which could cause the tool to warn on a clean build.

IP REPOS:       <redacted>/SLASH/linker/install.prj/iprepo

INFO: Creating new project 'slash' in '<redacted>/SLASH/linker/install.prj' ...
INFO: [IP_Flow 19-234] Refreshing IP repositories
WARNING: [IP_Flow 19-2248] Failed to load user IP repository '<redacted>/SLASH/linker/install.prj/iprepo'; Can't find the specified path.
If this directory should no longer be in your list of user repositories, go to the IP Settings dialog and remove it.

The fix checks config.ip_repository.exists() before appending, so the argument is only passed when the directory is actually present.