Only register Flight.proto with cargo if it exists

[tustvold]

Which issue does this PR close?

Closes #350.

Experimentation suggests if you "register" a file that doesn't exist with cargo:rerun-if-changed, it will always re-compile the crate. My hope is that this will resolve #350

[alamb]

Makes sense to me. What do you think @nevi-me ?

[codecov-commenter]

Codecov Report

Merging #351 (d5777ce) into master (488f826) will not change coverage.
The diff coverage is n/a.

❗ Current head d5777ce differs from pull request most recent head a729b6e. Consider uploading reports for the commit a729b6e to get more accurate results

@@           Coverage Diff           @@
##           master     #351   +/-   ##
=======================================
  Coverage   82.56%   82.56%           
=======================================
  Files         162      162           
  Lines       44063    44063           
=======================================
  Hits        36379    36379           
  Misses       7684     7684           

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[alamb]

The MIRI failure is unrelated to this PR: #345

[nevi-me]

The change makes sense, thanks @tustvold @alamb