Add garnix.io for caching and CI (#66)

https://garnix.io/

Pros:
* Caching for free
* Evaluation of checks, packages, devShells, etc. in parallel

Cons:
* In beta and it shows
* Logs seem to come a little slow in their web UI
* When builds finish, the “such and such derivations will be fetched,
such and such derivations will be built” output is erased, even though I
just saw it there...
* Only the last 100 lines of logs are saved (is this the case for
failures too...?)

flake.nix

@@ -20,6 +20,12 @@
     };
   };
 
+  nixConfig = {
+    extra-substituters = ["https://cache.garnix.io"];
+    extra-trusted-substituters = ["https://cache.garnix.io"];
+    extra-trusted-public-keys = ["cache.garnix.io:CTFPyKSLcx5RMJKfLo5EEPUObbA78b0YQ2DTCJXqr9g="];
+  };
+
   outputs = {
     self,
     nixpkgs,

garnix.yaml

@@ -0,0 +1,10 @@
+---
+# See: https://garnix.io/docs/yaml_config
+builds:
+  include:
+    - devShells.x86_64-linux.default
+    - devShells.aarch64-darwin.default
+    - packages.x86_64-linux.default
+    - packages.aarch64-darwin.default
+    - 'checks.x86_64-linux.*'
+    - 'checks.aarch64-darwin.*'