Update tar and use new safe unpacking function
The `unpack_in` function automatically handles the creation of the
directories and validates the path name, but it is only available
since version 0.4.11 of the `tar` crate.
This is the rustup side of fixing #1092.

Ensure that intermediate directories exist when unpacking an entry
The `unpack` function assumes that the directory in which the file is
being extracted exists, while most `tar` tools will automatically
create the intermediate directories if they are missing.
This would have avoided #1092.

Ensure that intermediate directories exist when unpacking an entry
The `unpack` function assumes that the directory in which the file is
being extracted exists, while most `tar` tools will automatically
create the intermediate directories if they are missing.
This would have avoided #1092.