Replace host cfg with exec (#688)

Something changed on Bazel CI (probably the version of buildifier) such that this suddenly broke on the latest nightly master despite us not changing anything

gazelle/modules_mapping/def.bzl

diff --git a/gazelle/modules_mapping/def.bzl b/gazelle/modules_mapping/def.bzl
index e90d454..04ea50f 100644
--- a/gazelle/modules_mapping/def.bzl
+++ b/gazelle/modules_mapping/def.bzl
@@ -37,7 +37,7 @@
             mandatory = True,
         ),
         "_generator": attr.label(
-            cfg = "host",
+            cfg = "exec",
             default = "//gazelle/modules_mapping:generator",
             executable = True,
         ),

python/packaging.bzl

diff --git a/python/packaging.bzl b/python/packaging.bzl
index d12a91e..5e79900 100644
--- a/python/packaging.bzl
+++ b/python/packaging.bzl
@@ -432,7 +432,7 @@
             ),
             "_wheelmaker": attr.label(
                 executable = True,
-                cfg = "host",
+                cfg = "exec",
                 default = "//tools:wheelmaker",
             ),
         },