Update cfg to exec (#137)

This makes this repo compatible with an upcoming bazel change removing
host here

subpar.bzl

diff --git a/subpar.bzl b/subpar.bzl
index 9146e00..a8ae7ef 100644
--- a/subpar.bzl
+++ b/subpar.bzl
@@ -134,7 +134,7 @@
     "compiler": attr.label(
         default = Label(DEFAULT_COMPILER),
         executable = True,
-        cfg = "host",
+        cfg = "exec",
     ),
     "compiler_args": attr.string_list(default = []),
     "zip_safe": attr.bool(default = True),