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

.gitlab-ci.yml

@@ -8,6 +8,9 @@ before_script:
   - pip install tox
 
 py3:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   before_script:
     - apt-get update
     - apt-get install texlive-latex-recommended -y
@@ -24,18 +27,30 @@ 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
 
 flake8:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   stage: tests
   script: tox -e flake8
 
 mypy:
+  except:
+    variables:
+      - $TRIGGERED_FROM_OTHER_PROJECT
   stage: tests
   script: tox -e mypy