test: only launch from-forge when triggered from another project

.gitlab-ci.yml

@@ -8,10 +8,16 @@ before_script:
   - pip install tox
 
 flake8:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   stage: tests
   script: tox -e flake8
 
 py3:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   stage: tests
   script: tox -e py3
 
@@ -20,14 +26,23 @@ py3-from-forge:
   script: tox -e py3-from-forge
 
 check-manifest:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   stage: tests
   script: tox -e check-manifest
 
 black:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   stage: tests
   script: tox -e black
 
 mypy:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   stage: tests
   script: tox -e mypy