feat: Support caching the blockmap file for the new version

[beyondkmp]

Currently, by default, both the new and old blockmap files are downloaded, resulting in double the CDN requests and traffic. Since the blockmap file is very small, we can keep a copy of the current version locally, so only the new version’s blockmap file needs to be downloaded, saving half of the CDN requests and traffic.

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Latest commit: ba1e1d7

The changes in this PR will be included in the next version bump.

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Name	Type
electron-updater	Patch

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[mmaietta]

we can keep a copy of the current version locally, so only the new version’s blockmap file needs to be downloaded

I think this is missing some edge cases, one of which is security-related I think?

• If the local "old"/current blockmap is manipulated before blockmap check, could that lead to a security vulnerability?
• Wondering if there's a previous reason as to why we want a "fresh" blockmap downloaded when checking for updates

electron-builder/packages/electron-updater/src/differentialDownloader/FileWithEmbeddedBlockMapDifferentialDownloader.ts

Lines 28 to 29 in 6a83607

	const dataBuffer = Buffer.allocUnsafe(sizeBuffer.readUInt32BE(0))
	await read(fd, dataBuffer, 0, dataBuffer.length, fileSize - sizeBuffer.length - dataBuffer.length)