build: Fix race in generating schema

[cgwalters]

This fixes the core bug causing builds to race right now; the
golang compiler might see a partially written file.

[cgwalters]

OK so this PR should fix the issues stalling other PRs; it will work without merging one of the other two options:

• build-sys: Require explicit make schema-update to regenerate #1308
• mantle: Remove generated cosa schema from git #1306

But after this lands we should pick one of those two.

[cgwalters]

Ohhh I see there are two generated files...

[cgwalters]

OK I think this one works. It's rather unfortunate I didn't hit these race conditions while building locally (I think my computer is too fast), nor in the original PR.

[mike-nguyen]

My computer must be slow because I hit this 😞 With the PR I can build cosa now.

/lgtm

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