Disable all warnings for homebrew & opam release builds

[gabelevi]

This should avoid issues like Homebrew/homebrew-core#14545 where our CI is happy with the build (since we use 4.03.0) but it hits a warning in 4.04.0 when built with homebrew.

Test Plan:

opam switch 4.04.0
eval `opam config env`
make clean
make # Errored
make clean
FLOW_RELEASE=1 make # Worked
make all-homebrew # Worked
opam pin add flowtype .

Previously make all-homebrew would error with 4.04.0

I could never get opam install flowtype to error, but I can at least confirm that this doesn't make it worse.

[gabelevi]

Just realized that the same warning is going to break opam too...let's see if I can fix that as well

[ilovezfs]

@gabelevi thanks for looking into this!

[avikchaudhuri]

@gabelevi This is the opam build PR: ocaml/opam-repository#9491

[gabelevi]

Well, it's good to know opam doesn't error, despite the warning. However, I distinctly remember it erroring on warnings before, hence #2237

[facebook-github-bot]

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