Fix "pmbootstrap config args /new/path"
Fix writing config options that are stored as CSV-list. Without this patch:
$ pmbootstrap config aports /tmp/pmaports
[23:43:58] Config changed: aports='/tmp/pmaports'
$ pmbootstrap config aports
[PosixPath('/'), PosixPath('t'), PosixPath('m'), PosixPath('p'), PosixPath('/'), PosixPath('p'), PosixPath('m'), PosixPath('a'), PosixPath('p'), PosixPath('o'), PosixPath('r'), PosixPath('t'), PosixPath('s')]
Storing strings as CSV list isn't great and we probably want to implement this more elegantly, see the related issue. But let's fix this first.
Related: #2412 (closed)
Merging this directly to work towards unblocking pmaports CI (#2411 (closed)).