This changes the bazel module dependencies so that the C++ jsonnet
repo dependency (which is actually used as the source for the
standard library and nothing else in the Bazel build) comes from
a specific commit of the jsonnet repo, rather than depending on
the 'published' jsonnet 0.20.0.
The two repos are tightly coupled anyway so I think this more
accurately reflects the situation. Particularly since the C++ repo
is also linked (at a specific commit) as a git submodule, which
is used for non-Bazel build and testing.