ARROW-7794: [Rust] [Flight] Remove hard-coded relative path to Flight.proto
ARROW-7794: [Rust] [Flight] Remove hard-coded relative path to Flight.proto

This is my second attempt at this and I made it more robust this time. I manually tested the following:

- `cargo publish --dry-run --allow-dirty`
- `cargo test` from `arrow/rust`
- `cargo test` from `arrow/rust/arrow-flight`
- Building another project that had a dependency on this branch

Closes #6873 from andygrove/ARROW-7794b

Authored-by: Andy Grove <f68909d51919859739a1c5d24450a2cc1026e93b@gmail.com>
Signed-off-by: Andy Grove <f68909d51919859739a1c5d24450a2cc1026e93b@gmail.com>

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