tests: pin OS version

Signed-off-by: Ilias Koutsakis <[email protected]>

Author: Lilykos

.github/workflows/ui-test.yml

@@ -13,7 +13,7 @@ on:
 jobs:
   UITest:
     name: UI Tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2

.github/workflows/web-api.yml

@@ -13,7 +13,7 @@ on:
 jobs:
   BackendSuite:
     name: Backend Tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     services:
       redis:
         image: redis