stdenv: Improve ELF detection for isELF

The isELF function only checks whether ELF is contained within the first
4 bytes of the file, which is a bit fuzzy and will also return
successful if it's a text file starting with ELF, for example:

  ELF headers
  -----------

  Some text here about ELF headers...

So instead, we're now doing a precise match on \x7fELF.

Signed-off-by: aszlig <aszlig@nix.build>
Acked-by: @Ericson2314
Closes: https://github.com/NixOS/nixpkgs/pull/47244

pkgs/stdenv/generic/setup.sh

@@ -211,7 +211,7 @@ isELF() {
     exec {fd}< "$fn"
     read -r -n 4 -u "$fd" magic
     exec {fd}<&-
-    if [[ "$magic" =~ ELF ]]; then return 0; else return 1; fi
+    if [ "$magic" = $'\177ELF' ]; then return 0; else return 1; fi
 }
 
 # Return success if the specified file is a script (i.e. starts with