[CI] Only run CI builds against nightly images

Author: Mordil

.gitlab/ci/centos-7.yml

@@ -11,7 +11,8 @@ include: '/.gitlab/ci/platform-test.yml'
 swift trunk:
   extends: .centos7
   image: swiftlang/swift:nightly-master-centos7
-  allow_failure: true
+  variables:
+    RUN_NIGHTLY: 1
 
 swift 5.5:
   extends: .centos7

.gitlab/ci/platform-test.yml

@@ -15,4 +15,10 @@ stages:
   script:
     - swift build --build-tests --enable-test-discovery --sanitize=thread -v 
     - swift test --skip-build
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "schedule" && $RUN_NIGHTLY'
+      allow_failure: true
+      when: always
+    - if: '$CI_PIPELINE_SOURCE != "schedule"'
+      when: always
 

.gitlab/ci/standard-platforms.yml

@@ -3,7 +3,8 @@ include: '/.gitlab/ci/platform-test.yml'
 swift trunk:
   extends: .platform-test
   image: swiftlang/swift:nightly-master-${SWIFT_PLATFORM_NAME}
-  allow_failure: true
+  variables:
+    RUN_NIGHTLY: 1
 
 swift 5.5:
   extends: .platform-test