tell infer it's running under buck

Summary:
Infer used to rely on env var `INFER_ARGS` containing `--buck` so as to know it's running under buck. This is no longer going to work in RE as env vars don't translate.

This is used in infer to clean up the output and make it suitable for caching.

This diff makes the flavour explicitly tell the infer invoked it's running under buck via a command line argument.

Reviewed By: cjhopman

fbshipit-source-id: 888a83fbcd5ed291281100e755a56e0894ae4dad

src/com/facebook/buck/cxx/CxxInferCaptureRule.java

@@ -209,6 +209,7 @@ protected ImmutableList<String> getShellCommandInternal(
                   //    that gets shipped as input to a RE job
                   .addAll(inferPlatform.getInferBin().getCommandPrefix(sourcePathResolver))
                   .add("capture")
+                  .add("--buck")
                   .add(
                       "--results-dir",
                       toAbsNormalizedPath(