fetchpatch: allow empty hash

Previously, when sha256 either wasn't defined or set to an empty string
fetchpatch would error out as follows:

'''
warning: found empty hash, assuming 'sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA='
...
/nix/store/agwlk2bcfvz2ggrsbvwd7696qj55frbi-stdenv-linux/setup: line 96: /build/: Is a directory
sed: couldn't flush stdout: Broken pipe
'''

This patch makes it show fetchurl's error message instead:

'''
warning: found empty hash, assuming 'sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA='
...
error: hash mismatch in fixed-output derivation:
         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
            got:    sha256-NWGWoyEgT/ztCwbhNgGPvG+nqX4bxtFnD+wds6fklbs=
'''

This is very convenient for TOFU.

Co-Authored-By: Ivar Scholten <ivar.scholten@protonmail.com>

pkgs/build-support/fetchpatch/default.nix

@@ -11,13 +11,9 @@ let
 in
 { stripLen ? 0, extraPrefix ? null, excludes ? [], includes ? [], revert ? false, ... }@args:
 
-let
-  # Make base-64 encoded SRI hash filename-safe using RFC 4648 §5
-  tmpname = lib.replaceStrings [ "+" "/" "=" ] [ "-" "_" "" ] args.sha256;
-in
 fetchurl ({
   postFetch = ''
-    tmpfile="$TMPDIR/${tmpname}"
+    tmpfile="$TMPDIR/patch"
     if [ ! -s "$out" ]; then
       echo "error: Fetched patch file '$out' is empty!" 1>&2
       exit 1