fix: bump cpp_jsonnet dependency to align with v0.20.0 release

[aidan-canva]

Context

Pulling the go-jsonnet v0.20.0 repository via Bazel and building locally results in an older version of std.libsonnet being bundled into the binary. As a result, enhancements that were made as part of v0.20.0 are not included.

It appears the cpp_jsonnet git submodule was bumped to the appropriate commit in https://github.com/google/jsonnet but the git hash and checksum from bazel/repositories.bzl was not.

Intent

Update bazel/repositories.bzl to use the correct v0.20.0 cpp_jsonnet dependency.

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