build: upgrade to golang 1.21.2

[holiman]

Upgrades to 1.21.2.

This PR also includes a tool to sanity-check the checksums in our checksum list. I expanded the semantics of the checksum txt file, so that a line starting with # https sets a new base-url, and we can download and verify each hash/file combo under the given base url.

Execute with go run ./build/ci.go sanitycheck.

Also, if we decide this is a nice idea, then we can make the fixtures-download use this scheme, and we don't have to have a fixtures-version in the main ci.go -- this part:

func downloadSpecTestFixtures(csdb *build.ChecksumDB, cachedir string) string {
	ext := ".tar.gz"
	base := "fixtures" // TODO(MariusVanDerWijden) rename once the version becomes part of the filename
	url := fmt.Sprintf("https://github.com/ethereum/execution-spec-tests/releases/download/v%s/%s%s", executionSpecTestsVersion, base, ext)

We could instead just infer it from the checksumdb.