tests: allow debugruntest tests to use a different Sapling binary

Summary: When testing D67711732, I noticed that in some cases there could be cases where debugruntest had new features but the older binary was still being used. We likely want to always use the latest debugruntest code, as it is mostly independent from Sapling.

Reviewed By: markbt

Differential Revision: D67870324

fbshipit-source-id: cc832a612501cdbc4d0247c00deedecce2f9fa6e

Author: sggutier

eden/scm/tests/run-tests.py

@@ -2357,8 +2357,10 @@ def _run(self, env):
         if use_edenfs:
             self._edenfsmanager.start(env)
 
+        cmd = env.get("HGTEST_DEBUGRUNTEST_HG", self._hgcommand)
+
         cmdargs = [
-            self._hgcommand,
+            cmd,
             "debugpython",
             "--",
             "-m",