style: move push action to new dir so github and gitea can share it

.actions/init/action.yaml

@@ -0,0 +1,21 @@
+name: "initialize"
+description: "install nix & use cachix"
+
+inputs:
+  token:
+    description: "cachix auth token"
+    required: true
+
+runs:
+  using: "composite"
+  steps:
+    - name: install nix
+      uses: cachix/install-nix-action@v31
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+
+    - name: use cachix
+      uses: cachix/cachix-action@v16
+      with:
+        name: trevstack
+        authToken: "${{ inputs.token }}"