chore(tvix/store): use lazy_static and dummy digests

While there's currently nothing in here checking the size of the digest,
we should use something that passes the to-be-introduced validate()
function.

Change-Id: I0c515d9e3afc79292dedebce659a32485aa3d936
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7649
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 blake3 = { version = "1.3.1", features = ["rayon", "std"] }
+lazy_static = "1.4.0"
 prost = "0.11.2"
 tonic = "0.8.2"