unarchive: workaround for tar that does not support "-C"

Fixes #7777

*Only when tar -C is not supported*: Set CWD instead.

The reason I made this workaround conditional, instead of applying it in all cases as a "lowest common denominator", is that on systems with GNU tar the "-C" option is quite well known and commonly used. In particular, when examing the output of "ansible -v", it is easier to understand and test what ansible is doing when a single self-contained tar command is being executed.

But it could have gone either way, and I'm open to other opinions.