arcanist: add python3 to fix 'arc anoid'

Patch out the requirement from 'arc' that it needs 'python3' in
$PATH at runtime, because once built the python script will have
a fixed absolute path shebang to python3. (The check makes sense
outside of Nix though.)

Not injecting python3 in PATH prevents leaking python3 into the
environment in which arc runs linters etc.

pkgs/development/tools/misc/arcanist/default.nix

@@ -6,6 +6,7 @@
 , lib, stdenv
 , installShellFiles
 , which
+, python3
 }:
 
 # Make a custom wrapper. If `wrapProgram` is used, arcanist thinks .arc-wrapped is being
@@ -33,7 +34,9 @@ stdenv.mkDerivation {
     sha256 = "0jiv4aj4m5750dqw9r8hizjkwiyxk4cg4grkr63sllsa2dpiibxw";
   };
 
-  buildInputs = [ php ];
+  patches = [ ./dont-require-python3-in-path.patch ];
+
+  buildInputs = [ php python3 ];
 
   nativeBuildInputs = [ bison flex installShellFiles ];
 

pkgs/development/tools/misc/arcanist/dont-require-python3-in-path.patch

@@ -0,0 +1,26 @@
+Don't require python3 in PATH
+
+Once packaged, the arcanoid.py script has an absolute path shebang to
+python3, so there is no need to also require python3 in PATH.
+
+This prevents leaking in a python3 in PATH in the environment which arc
+runs linters etc.
+
+Author: bjorn.forsman@gmail.com
+diff -uNr arcanist.orig/src/workflow/ArcanistAnoidWorkflow.php arcanist.new/src/workflow/ArcanistAnoidWorkflow.php
+--- arcanist.orig/src/workflow/ArcanistAnoidWorkflow.php	2022-03-31 13:23:30.865095192 +0200
++++ arcanist.new/src/workflow/ArcanistAnoidWorkflow.php	2022-04-01 12:19:15.644159639 +0200
+@@ -24,13 +24,6 @@
+   }
+ 
+   public function runWorkflow() {
+-    if (!Filesystem::binaryExists('python3')) {
+-      throw new PhutilArgumentUsageException(
+-        pht(
+-          'The "arc anoid" workflow requires "python3" to be available '.
+-          'in your $PATH.'));
+-    }
+-
+     $support_dir = phutil_get_library_root('arcanist');
+     $support_dir = dirname($support_dir);
+     $support_dir = $support_dir.'/support/';