ci: add the NIXPKGS_CHANNEL shortcut

.gitlab-ci.yml

@@ -7,19 +7,17 @@ nixos-unstable:
   stage: build
   script: nix-shell --run ./.gitlab-ci.sh
   variables:
-    NIX_PATH: nixpkgs=channel:nixos-unstable
+    NIXPKGS_CHANNEL: nixos-unstable
     IMAGE_TAG: latest
 
 nixos-19.03:
   stage: build
   script: nix-shell --run ./.gitlab-ci.sh
   variables:
-    NIX_PATH: nixpkgs=channel:nixos-19.03
-    IMAGE_TAG: nixos-19.03
+    NIXPKGS_CHANNEL: nixos-19.03
 
 nixos-19.09:
   stage: build
   script: nix-shell --run ./.gitlab-ci.sh
   variables:
-    NIX_PATH: nixpkgs=channel:nixos-19.09
-    IMAGE_TAG: nixos-19.09
+    NIXPKGS_CHANNEL: nixos-19.09

.travis.sh

@@ -7,12 +7,14 @@ set -euo pipefail
 ./build
 
 # default to Docker Hub
+# the user has to set REGISTRY_USER and REGISTRY_PASSWORD
 : "${REGISTRY:=docker.io}"
 : "${IMAGE_PREFIX:=nixpkgs}"
 
-# IMAGE_TAG is provided by .travis.yml
-
-# the user has to set REGISTRY_USER and REGISTRY_PASSWORD
+# either pass those two or set the NIXPKGS_CHANNEL
+: "${IMAGE_TAG:=$NIXPKGS_CHANNEL}"
+: "${NIX_PATH:=nixpkgs=channel:$NIXPKGS_CHANNEL}"
+export NIX_PATH
 
 if [[ "$TRAVIS_BRANCH" = master && -z "$TRAVIS_PULL_REQUEST_BRANCH" ]]; then
   ./docker-login "$REGISTRY_USER" "$REGISTRY_PASSWORD" "$REGISTRY"

.travis.yml

@@ -5,11 +5,11 @@ nix: 2.2.1
 matrix:
   include:
   - name: nixos-unstable
-    env: NIX_PATH=nixpkgs=channel:nixos-unstable IMAGE_TAG=latest
+    env: NIXPKGS_CHANNEL=nixos-unstable IMAGE_TAG=latest
   - name: nixos-19.03
-    env: NIX_PATH=nixpkgs=channel:nixos-19.03 IMAGE_TAG=nixos-19.03
+    env: NIXPKGS_CHANNEL=nixos-19.03
   - name: nixos-19.09
-    env: NIX_PATH=nixpkgs=channel:nixos-19.09 IMAGE_TAG=nixos-19.09
+    env: NIXPKGS_CHANNEL=nixos-19.09
 
 script:
   - nix-shell --run ./.travis.sh