5x faster unit test compilation on nix dev env (#467)

* 5x faster unit test compilation

Both on a warm cache running the command:
time cargo test test_pk_recovery

Before:
cargo test test_pk_recovery  635.13s user 26.51s system 679% cpu 1:37.38 total

After:
cargo test test_pk_recovery  45.39s user 11.66s system 280% cpu 20.361 total

I should probably find out why our compile cache keeps getting
invalidated, but this is a quick win. Cost is this adds in a new native dependency.

* Actually let's just keep this in the nix dev env

flake.nix

@@ -18,6 +18,11 @@
       in {
         devShell = pkgs.mkShell {
 
+          env = {
+            SCCACHE_GHA_ENABLED = true;
+            RUSTC_WRAPPER = "sccache";
+          };
+
           # Everything in this list is added to your path
           buildInputs =
             with pkgs;
@@ -30,6 +35,7 @@
               # Development
               cargo-watch
               cargo-audit
+              sccache
             ] ++
 
             pkgs.lib.optionals pkgs.stdenv.isDarwin