Replace cfg = "host" with cfg = "exec"

This is a no-op cleanup, 'host' is being renamed to 'exec', see:
bazelbuild/bazel#17032

Author: cushon

gax-java/gax_java.bzl

@@ -77,7 +77,7 @@ google_java_format_verification = rule(
         "srcs": attr.label_list(allow_files = True),
         "formatter": attr.label(
             executable = True,
-            cfg = "host",
+            cfg = "exec",
         ),
     },
     outputs = {"output_file": "%{name}.txt"},