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

pkgs/build-support/setup-hooks/patch-shebangs.sh

@@ -51,7 +51,7 @@ patchShebangs() {
         isScript "$f" || continue
 
         read -r oldInterpreterLine < "$f"
-        read -r oldPath arg0 args <<< "${oldInterpreterLine:3}"
+        read -r oldPath arg0 args <<< "${oldInterpreterLine:2}"
 
         if [[ -z "$pathName" ]]; then
             if [[ -n $strictDeps && $f == "$NIX_STORE"* ]]; then