fetchfossil: support SRI hashes

pkgs/build-support/fetchfossil/default.nix

···
 {stdenv, lib, fossil, cacert}:
 
-{name ? null, url, rev, sha256}:
+{ name ? null
+, url
+, rev
+, sha256 ? ""
+, hash ? ""
+}:
 
+if hash != "" && sha256 != "" then
+  throw "Only one of sha256 or hash can be set"
+else
 stdenv.mkDerivation {
   name = "fossil-archive" + (lib.optionalString (name != null) "-${name}");
   builder = ./builder.sh;
···
   # https://www.fossil-scm.org/index.html/doc/trunk/www/env-opts.md
   impureEnvVars = [ "http_proxy" ];
 
-  outputHashAlgo = "sha256";
+  outputHashAlgo = if hash != "" then null else "sha256";
   outputHashMode = "recursive";
-  outputHash = sha256;
+  outputHash = if hash != "" then
+    hash
+  else if sha256 != "" then
+    sha256
+  else
+    lib.fakeSha256;
 
   inherit url rev;
   preferLocalBuild = true;