fix(Config Schema): Safe generation of standalone AJV validator

[pgrzesik]

As noticed by @medikoo, current implementation of saving the file might not be safe and prone to race conditions.

Addresses: #10679

[codecov]

Codecov Report

Merging #10680 (f7db43c) into main (4201b30) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head f7db43c differs from pull request most recent head 4ef79d5. Consider uploading reports for the commit 4ef79d5 to get more accurate results

@@           Coverage Diff           @@
##             main   #10680   +/-   ##
=======================================
  Coverage   86.42%   86.42%           
=======================================
  Files         300      300           
  Lines       12398    12402    +4     
=======================================
+ Hits        10715    10719    +4     
  Misses       1683     1683           

Impacted Files	Coverage Δ
...sses/config-schema-handler/resolve-ajv-validate.js	88.88% <100.00%> (+1.08%)	⬆️

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[medikoo]

I think it'll be cleaner to follow same approach as in some other places as:

1. Get process-specific tmpdir via process-utils/tmpdir
2. Write file with regular fsp.writeFile to it
3. Safely move a file

We do it when handling deprecation report:

serverless/lib/utils/log-deprecation.js

Lines 141 to 147 in 906f42f

	const tmpHealthStatusFilename = path.resolve(await resolveTmpdir(), 'health-status');
	await Promise.all([
	fse.ensureDir(path.dirname(healthStatusFilename)),
	fsp.writeFile(tmpHealthStatusFilename, healthStatus.join('\n')),
	]);
	await safeMoveFile(tmpHealthStatusFilename, healthStatusFilename);

(still here we do not need to ensure target dir, as it's already ensured by ensureDir)

[medikoo]

Looks great 👍

[perrin4869]

I was encountering this bug when trying to upgrade to v3, thanks for solving it!!!

[pgrzesik]

Sorry for the trouble @perrin4869 - hopefully this fix will address this problem 👍