So, on Ubuntu 21 (the one I have installed), you can actually install a
   bazel binary from the signed repositories.  Maybe earlier ubuntus too,
   I dunno.
   # installs version 3.5.1 of the standalone bazel build tool, for me
   $ sudo apt-get install bazel-bootstrap
   Still, it seems incredibly rude of tensorflow to use only such a rare
   build tool for their project with no reasonable explanation.  Bazel
   doesn't even support 32 bit systems.  It is not hard to put all the
   sourcefiles into cmake or autotools, for something so widely used.
   git clone [1]https://github.com/mozilla/DeepSpeech.git
   cd DeepSpeech
   git checkout v0.9
   git log
   commit ab714a8d1b8aecc92c0a96f85e958e1a617a2f4b (HEAD -> r0.9,
   origin/r0.9)
   Merge: 13a490d1 85d057e1
   Author: lissyx <[2]1645737+lissyx@users.noreply.github.com>
   Date:   Wed Apr 7 18:35:56 2021 +0200
   git submodule update --init --recursive
   ... long time passes while the entire tensorflow history is downloaded
   ... I think there are flags to do it faster if needed.  Could also
   check .gitmodules and link the module repository to another tensorflow
   clone, to skip it.

References

   1. https://github.com/mozilla/DeepSpeech.git
   2. mailto:1645737+lissyx@users.noreply.github.com