examples: switch bison to the bazel mirror (#1521)

The ftpmirror.gnu.org redirect service is unreliable and has been
timing out in CI, breaking every build that touches bison or mesa.

Switch to the bazel.build mirror of the xz archive instead.

Author: novas0x2a

examples/third_party/bison/bison_repositories.bzl

@@ -11,7 +11,7 @@ def bison_repositories():
         build_file = Label("//bison:BUILD.bison.bazel"),
         strip_prefix = "bison-3.8.2",
         urls = [
-            "https://ftpmirror.gnu.org/gnu/bison/bison-3.8.2.tar.gz",
+            "https://mirror.bazel.build/ftp.gnu.org/gnu/bison/bison-3.8.2.tar.xz",
         ],
-        integrity = "sha256-BsnhO99+sk1M62tZIFpPZ8LH5yExGWREMP6C+9FKCrs=",
+        integrity = "sha256-m7oCFMz38QecXVkhAEUie89hlRmEDr+oDNOEnP9aW/I=",
     )