ci: add build from main, split stages properly

.gitlab-ci.yml

@@ -1,7 +1,31 @@
-build:
+stages:
+  - instantiate
+  - build
+
+instantiate 20.09:
+  tags:
+    - nix
+  stage: instantiate
+  script:
+    - nix-instantiate -I nixpkgs=https://github.com/hexchen/nixpkgs/archive/hexchen-20.09.tar.gz -A deploy.all
+
+instantiate main:
+  tags:
+    - nix
+  stage: instantiate
+  script:
+    - nix-instantiate -I nixpkgs=https://github.com/hexchen/nixpkgs/archive/hexchen-main.tar.gz -A deploy.all
+
+build 20.09:
   tags:
     - nix
   stage: build
   script:
-    - nix-instantiate -I nixpkgs=https://github.com/hexchen/nixpkgs/archive/hexchen-20.09.tar.gz -A deploy.all
     - nix-build -A deploy.all -I nixpkgs=https://github.com/hexchen/nixpkgs/archive/hexchen-20.09.tar.gz
+
+build main:
+  tags:
+    - nix
+  stage: build
+  script:
+    - nix-build -A deploy.all -I nixpkgs=https://github.com/hexchen/nixpkgs/archive/hexchen-main.tar.gz