.github: Switched automated testing on GitHub to use omnetpp-6.x vers…

…ion.

Previously it used the master branch which is basically omnetpp-7.x, but that version is into the future.

.github/workflows/build-linux.yml

@@ -25,6 +25,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet

.github/workflows/chart-tests.yml

@@ -20,6 +20,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet

.github/workflows/feature-tests.yml

@@ -22,6 +22,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet

.github/workflows/fingerprint-tests.yml

@@ -23,6 +23,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet

.github/workflows/module-tests.yml

@@ -21,6 +21,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet

.github/workflows/other-tests.yml

@@ -22,6 +22,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet

.github/workflows/statistical-tests.yml

@@ -20,6 +20,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet

.github/workflows/unit-tests.yml

@@ -21,6 +21,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet

.github/workflows/validation-tests.yml

@@ -20,6 +20,7 @@ jobs:
         with:
           repository: omnetpp/omnetpp
           path: omnetpp
+          ref: omnetpp-6.x
       - uses: actions/checkout@v4
         with:
           path: inet