fix silent timeout, set build timeout

using `--option` like this hid that the silent timeout was never
actually set, instead we set the unknown and thus ignored option
`--max-silent-time`. while we're at it we can also set a timeout
for the entire build, chosen as two hours because that should be
enough for all current jobs (and hopefully it'll stay that way).

buildbot_nix/__init__.py

@@ -517,10 +517,10 @@ def nix_build_config(
                 "true",
                 # do not build directly on the coordinator
                 "--max-jobs", "0",
-                "--option",
                 # stop stuck builds after 20 minutes
-                "--max-silent-time",
-                str(60 * 20),
+                "--max-silent-time", str(60 * 20),
+                # kill builds after two hours regardless of activity
+                "--timeout", "7200",
                 "--out-link",
                 util.Interpolate("result-%(prop:attr)s"),
                 util.Interpolate("%(prop:drv_path)s^*"),