Do not rely on hacky solution for checksum validation

[kamilogorek]

The solution here is not great, and it would be nice to have something more reliable, as this was already a fix for previously happening incident:

craft/src/targets/github.ts

Lines 398 to 404 in 477308a

	// XXX: This is a bit hacky as we rely on various things:
	// 1. GitHub issuing a redirect to AWS S3.
	// 2. S3 using the MD5 hash of the file for its ETag cache header.
	// 3. The file being small enough to fit in memory.
	//
	// Note that if assets are large (5GB) assumption 2 is not correct. See
	// https://github.com/getsentry/craft/issues/322#issuecomment-964303174

Original issues: #308 #328
Jira ref: FUNT-22