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ARROW-7003: [Rust] Generate flatbuffers files in docker build image

This PR updates the build docker image to generate IPC source based on the flatbuffer specification.

Note that the generated files are currently not used in the build (the generated files have a `_generated.rs` suffix so don't overwrite the checked in versions of the files).

We cannot fully automate this until https://github.com/google/flatbuffers/issues/5589 is resolved, or we figure out a workaround, but I suggest we merge this current PR since it lays the groundwork.

Closes #5738 from andygrove/ARROW-7003 and squashes the following commits:

80abddb9c <Andy Grove> lint
43f3e8931 <Andy Grove> reset workdir
0a186b5ae <Andy Grove> lint
6a9c6f04c <Andy Grove> fix docker lint issue
b2c9173ce <Andy Grove> Generate flatbuffers files in docker build image

Authored-by: Andy Grove <andygrove73@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>