allow buildtime publisher to run without project root

Summary:
This is against the common case of buck2, let's fix up the implementation to
support the default style of relative paths.

Reviewed By: justintrudell

Differential Revision: D59920099

fbshipit-source-id: d033102cf758e5d2e0c0b586ca47d7e8b0ddc547

prelude/genrule.bzl

@@ -42,7 +42,6 @@ _BUILD_ROOT_LABELS = {label: True for label in [
     "flowtype_ota_safety_target",  # produces JSON containing file paths that are project-relative
     "ctrlr_setting_paths",
     "llvm_buck_genrule",
-    "cm_buildtime_publisher",  # does a 'buck2 run' of a CM publisher target
 ]}
 
 # In Buck1 the SRCS environment variable is only set if the substring SRCS is on the command line.