fix: pyc files in Meson mesonbuild repo causing rebuilds

Fixes bazel-contrib#1342

It seems that pyc files appear in the Meson repo between the first and
second time a python toolchain is used and if these are not excluded,
the new pyc files invalidate dependent rules, forcing a rebuild.

For more context, see bazel-contrib#1342 and
https://bazelbuild.slack.com/archives/CA306CEV6/p1734196424601509

toolchains/built_toolchains.bzl

@@ -20,7 +20,9 @@ exports_files(["meson.py"])
 
 filegroup(
     name = "runtime",
-    srcs = glob(["mesonbuild/**"]),
+    # NOTE: excluding __pycache__ is important to avoid rebuilding due to pyc
+    # files, see https://github.com/bazel-contrib/rules_foreign_cc/issues/1342
+    srcs = glob(["mesonbuild/**"], exclude = ["**/__pycache__/*"]),
     visibility = ["//visibility:public"],
 )
 """