feat(users/firefly): initialize with key for as

Change-Id: I7da98cd442d4dcd9fdecb8b5030f8681a2f03cd5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2012
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/firefly/OWNERS

@@ -0,0 +1,3 @@
+inherited: false
+owners:
+  - firefly

users/firefly/keys.nix

@@ -0,0 +1,7 @@
+{ ... }:
+
+rec {
+  as = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIN9i8fs10/BjNEqFXD+3fQeQ0SuHnQx4WpuqUg4caeed firefly@as";
+
+  whitby = [ as ];
+}