FileExistsError during second cylc install with --directory option

[MetRonnie]

Describe the bug

I get traceback when doing a second cylc install due to a problem with how the _cylc-install/source symlink is handled.

Release version(s) and/or repository branch(es) affected?

master

Steps to reproduce the bug

1. Install a workflow, e.g. the cylc-doc forecasting workflow

   $ cd ~/github/cylc-doc
   $ cylc install -C cylc/doc/etc/tutorial/cylc-forecasting-workflow/
   INSTALLED cylc-forecasting-workflow/run1 from cylc/doc/etc/tutorial/cylc-forecasting-workflow

2. (Optional - run the workflow, which is what I did but probably isn't needed to reproduce)

3. Install the second run, and get FileExistsError

   $ cylc install -C cylc/doc/etc/tutorial/cylc-forecasting-workflow/
   Traceback (most recent call last):
   File "~/miniconda3/envs/cylc8/bin/cylc", line 33, in <module>
       sys.exit(load_entry_point('cylc-flow', 'console_scripts', 'cylc')())
   File "~/github/cylc-flow/cylc/flow/scripts/cylc.py", line 538, in main
       execute_cmd(command, *cmd_args)
   File "~/github/cylc-flow/cylc/flow/scripts/cylc.py", line 174, in execute_cmd
       COMMANDS[cmd].resolve()(*args)
   File "~/github/cylc-flow/cylc/flow/terminal.py", line 260, in wrapper
       wrapped_function(*wrapped_args, **wrapped_kwargs)
   File "~/github/cylc-flow/cylc/flow/scripts/install.py", line 187, in main
       install(parser, opts, reg)
   File "~/github/cylc-flow/cylc/flow/scripts/install.py", line 232, in install
       cli_symlink_dirs=cli_symdirs
   File "~/github/cylc-flow/cylc/flow/workflow_files.py", line 1354, in install_workflow
       source_link.symlink_to(source)
   File "~/miniconda3/envs/cylc8/lib/python3.7/pathlib.py", line 1309, in symlink_to
       self._accessor.symlink(target, self, target_is_directory)
   File "~/miniconda3/envs/cylc8/lib/python3.7/pathlib.py", line 418, in symlink
       return os.symlink(a, b)
   FileExistsError: [Errno 17] File exists: 'cylc/doc/etc/tutorial/cylc-forecasting-workflow' -> '~/cylc-run/cylc-forecasting-workflow/_cylc-install/source'

Additional context

The source symlink is broken (and was already broken after the first install)

$ readlink ~/cylc-run/cylc-forecasting-workflow/_cylc-install/source
cylc/doc/etc/tutorial/cylc-forecasting-workflow

Looks like the relative path needs to be expanded to an absolute one, but this might be a slightly separate problem to the main issue?

Pull requests welcome!
This is an Open Source project - please consider contributing a bug fix
yourself (please read CONTRIBUTING.md before starting any work though).

@datamel I'm happy to have a go at fixing this myself, but can I ask you to try and reproduce?