Auto merge of #78409 - pietroalbini:build-manifest-checksum-cache, r=Mark-Simulacrum

Add checksums cache to build-manifest

During the release process we're currently calculating the SHA256 of each file three times:

1. In `build-manifest`, to fill the `hash = "f00"` keys of the manifests.
2. In `promote-release`, to generate the `.sha256` files.
3. In `promote-release`, to generate the `.asc` GPG signatures.

Calculations 1. and 2. could be merged into a single one if there was a way for `build-manifest` to pass the checksums it generated over to `promote-release`. Unfortunately calculation 3. can't be merged as GPG requires extra metadata to be hashed.

This PR adds support for merging 1. and 2. by creating the `BUILD_MANIFEST_CHECKSUM_CACHE` environment variable, which points to a JSON file storing a cache of all the calculated checksums. `build-manifest` will load it at startup and avoid generating existing checksums, and it will dump its internal checksums cache into it when it exits successfully.

This PR also allows to run `build-manifest` multiple times without the need to wait for checksums to be calculated in the following invocations. The speedup will allow to work torwards a fix for https://github.com/rust-lang/promote-release/issues/15 without impacting the release process duration nor our storage costs.

This PR can be reviewed commit-by-commit.
r? `@Mark-Simulacrum`

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