stdenv/patchShebangs: fix shebang check

patchShebangs has a bug that shows itself on files that have the
executable bit set but have no shebang (i.e. a blank/empty first line).
The shell would then evaluate this:

if [ != '#!' ]; then
  # not evaluated
fi

With proper quoting we get the correct behaviour:

if [ "" != '#!' ]; then
  # this will be evaluated
fi

pkgs/stdenv/generic/setup.sh

@@ -669,7 +669,7 @@ patchShebangs() {
     local newInterpreterLine
 
     for f in $(find "$dir" -type f -perm +0100); do
-        if [ $(head -1 "$f" | head -c +2) != '#!' ]; then
+        if [ "$(head -1 "$f" | head -c +2)" != '#!' ]; then
             # missing shebang => not a script
             continue
         fi