pristine-xz: Fixes for Bug 962354

I found two issues in the code related to #962354:

1. The code would incorrectly detect a pixz tar index for any stream with multiple blocks due to operator precedence rules when mixing and with =.
2. The --block-list= argument would be added before the command name in the argument array, causing the invocation to fail.

I'm embarrassed to admit that both of these issues were introduced by me in !3 (merged). Glad to get the chance to fix them.

Thanks for considering, Kevin