gitlab-ci: Add support for builds against stable branch

.gitlab-ci.yml

@@ -34,7 +34,43 @@ variables:
   # GHC during the package builds. This is instantiated with, e.g., -dcore-lint
   # during GHC validation builds.
 
-build:
+# A build triggered from a ghc/ghc> pipeline.
+build-pipeline:
+  extends: .build
+  before_script:
+    - |
+      if [ -n "$GHC_PIPELINE_ID" ]; then
+        job_name="validate-x86_64-linux-fedora27"
+        job_id=$(nix run -f scripts/build-all.nix find-job \
+          --arg bindistTarball $GHC_TARBALL \
+          -c find-job.sh $GHC_PROJECT_ID $GHC_PIPELINE_ID $job_name)
+        echo "Pulling ${job_name} binary distribution from Pipeline $GHC_PIPELINE_ID (job $job_id)..."
+      fi
+  only:
+    - api
+    - pipelines
+    - triggers
+    - web
+
+# Build against the master branch
+build-master:
+  extends: .build
+  variables:
+    GHC_TARBALL: "https://gitlab.haskell.org/api/v4/projects/1/jobs/artifacts/master/raw/ghc-x86_64-fedora27-linux.tar.xz?job=validate-x86_64-linux-fedora27"
+  only:
+    - branches
+    - merge_requests
+
+# Build against the 8.8 branch
+build-8.8:
+  extends: .build
+  variables:
+    GHC_TARBALL: "https://gitlab.haskell.org/api/v4/projects/1/jobs/artifacts/ghc-8.8/raw/ghc-8.8.1-x86_64-unknown-linux.tar.xz?job=validate-x86_64-linux-fedora27"
+  only:
+    - branches
+    - merge_requests
+
+.build:
   stage: test
 
   tags:
@@ -44,20 +80,11 @@ build:
   image: nixos/nix
 
   cache:
-    key: build-all
+    key: build-HEAD
     paths:
       - store.nar
 
   script:
-    - |
-      if [ -n "$GHC_PIPELINE_ID" ]; then
-        job_name="validate-x86_64-linux-fedora27"
-        job_id=$(nix run -f scripts/build-all.nix find-job \
-          --arg bindistTarball $GHC_TARBALL \
-          -c find-job.sh $GHC_PROJECT_ID $GHC_PIPELINE_ID $job_name)
-        echo "Pulling ${job_name} binary distribution from Pipeline $GHC_PIPELINE_ID (job $job_id)..."
-      fi
-
     - echo "Bindist tarball is $GHC_TARBALL"
     - |
       nix build \
@@ -115,7 +142,7 @@ update-repo:
     - cp summary.dot.svg repo
 
   dependencies:
-    - build
+    - build-master
 
   after_script:
     - rm -Rf keys