systemd-run: refuse --working-directory option with --scope

systemd-run: refuse --working-directory option with --scope

systemd-run's man page says the following about the working directory of
the process:

"If a command is run as transient scope unit, it will be executed
by systemd-run itself as parent process and will thus inherit the
execution environment of the caller."

This means working directory assignment does not work, as evidenced by
the following invocation:
```bash
$ systemd-run --scope --property=WorkingDirectory=/tmp/ bash -c 'echo $(pwd)'
Unknown assignment: WorkingDirectory=/tmp/
```

However, using the shorthand switch --working-directory silently ignores
this instead of giving a similar error.
```bash
systemd-run --scope --user --working-directory=/tmp/ bash -c 'echo $(pwd)'
Running scope as unit: run-r19cc32e744e64285814dbf2204637a2b.scope
/home/test/projects/systemd
```

This commit fixes this by explicitly generating an error instead of
silently ignoring the switch:
```bash
$ systemd-run --scope --working-directory=/tmp/ bash -c 'echo $(pwd)'
--working-directory is not supported in --scope mode.
```