Disable MacOS and Windows tests for forks

Forks do not have access to the group runners, so their pipelines will
get stuck. By disabling the MacOS and Windows tests we can ensure the
pipelines at least report a proper status.

.gitlab/ci/macos.yml

 ---
+.only-source-repository:
+  only:
+    - branches@inko-lang/inko
+    - tags@inko-lang/inko
+
 test:compiler:macos:
+  extends: .only-source-repository
   stage: test
   tags:
     - macos
@@ -17,6 +23,7 @@ test:compiler:macos:
       - compiler/vendor/ruby
 test:ivm:macos:
+  extends: .only-source-repository
   stage: test
   tags:
     - macos
@@ -33,6 +40,7 @@ test:ivm:macos:
       - vm/target
 test:runtime:macos:
+  extends: .only-source-repository
   stage: test-runtime
   tags:
     - macos
@@ -53,6 +61,7 @@ test:runtime:macos:
     - test:ivm:macos
 release:compiled:macos:
+  extends: .only-source-repository
   stage: release
   tags:
     - macos

.gitlab/ci/windows.yml

 ---
+.only-source-repository:
+  only:
+    - branches@inko-lang/inko
+    - tags@inko-lang/inko
+
 test:compiler:windows:
+  extends: .only-source-repository
   stage: test
   tags:
     - windows
@@ -17,6 +23,7 @@ test:compiler:windows:
       - compiler/vendor/ruby
 test:ivm:windows:
+  extends: .only-source-repository
   image: windows:1809
   stage: test
   tags:
@@ -34,6 +41,7 @@ test:ivm:windows:
       - vm/target
 test:runtime:windows:
+  extends: .only-source-repository
   stage: test-runtime
   tags:
     - windows
@@ -54,6 +62,7 @@ test:runtime:windows:
     - test:ivm:windows
 release:compiled:windows:
+  extends: .only-source-repository
   stage: release
   tags:
     - windows