Upgrade bazel to 4.0.0

It's released: https://github.com/bazelbuild/bazel/releases/tag/4.0.0
No need to use the release candidate anymore.

Unfortunately we still need our @platforms entry in WORKSPACE because
the version that ships with bazel is still over a year old.

I generated the file like so:

    $ wget 'https://github.com/bazelbuild/bazel/releases/download/4.0.0/bazel-4.0.0-linux-x86_64'
    $ xz bazel-4.0.0-linux-x86_64
    $ mv bazel-4.0.0-linux-x86_64.xz bazel_4.0.0.xz

Change-Id: I7fdef450ba778b425554c4eb1a0a8bb062c35f6e
diff --git a/tools/bazel b/tools/bazel
index a941938..4054da3 100755
--- a/tools/bazel
+++ b/tools/bazel
@@ -24,7 +24,7 @@
   exec "${BAZEL_OVERRIDE}" "$@"
 fi
 
-readonly VERSION="4.0.0rc6-202012121845+159e00701d"
+readonly VERSION="4.0.0"
 
 readonly DOWNLOAD_DIR="${HOME}/.cache/bazel"
 # Directory to unpack bazel into.  This must change whenever bazel changes.