patch-shebangs: use isScript to safely check for shebang start

Fixes commonly encountered errors about broken pipes or null-bytes in
command-substitution.

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

@@ -19,9 +19,8 @@ patchShebangs() {
     local newInterpreterLine
 
     find "$dir" -type f -perm -0100 | while read f; do
-        if [ "$(head -1 "$f" | head -c+2)" != '#!' ]; then
-            # missing shebang => not a script
-            continue
+        if ! isScript "$f"; then
+          continue
         fi
 
         oldInterpreterLine=$(head -1 "$f" | tail -c+3)