Andrew Childs ca156a66b7 stdenv/patchShebangs: fix off by one reading old interpreter
This caused shebangs that were already store paths to be rewritten.

Introduced by ab4c35982269e8f1f15e07afa7fedf65c749f276 in #94642

Example difference:

    $ echo "hello world" | tail -c+3
    llo world

    $ str="hello world"; echo ${str:3}
    lo world
2021-02-09 13:05:38 +09:00
..
2020-11-26 14:06:07 +01:00
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2017-12-19 01:45:42 +00:00