fix: replace cfg = "host" with cfg = "exec" (#2637)

This is a no-op cleanup, 'host' is being renamed to 'exec', see: https://togithub.com/bazelbuild/bazel/issues/17032

Fixes #2514 ☕️

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"},

rules_java_gapic/java_gapic.bzl

@@ -94,7 +94,7 @@ _java_gapic_postprocess_srcjar = rule(
         "formatter": attr.label(
             default = Label("//:google_java_format_binary"),
             executable = True,
-            cfg = "host",
+            cfg = "exec",
         ),
     },
     outputs = {
@@ -152,7 +152,7 @@ _java_gapic_samples_srcjar = rule(
         "formatter": attr.label(
             default = Label("//:google_java_format_binary"),
             executable = True,
-            cfg = "host",
+            cfg = "exec",
         ),
     },
     outputs = {