GitHub actions: Update scons-pot workflow to set locale to 'en_US.UTF-8'

=> Fix Gettext error due to "coding: UTF-8" removals in python files

.github/workflows/scons-pot.yml

@@ -3,7 +3,10 @@ on:
   push:
     branches:
       - master
-
+
+env:
+  LANG: "en_US.UTF-8"
+  LC_ALL: "en_US.UTF-8"
 jobs:
   scons-pot: 
     runs-on: ubuntu-latest
@@ -16,6 +19,8 @@ jobs:
           python-version: 3.7
       - name: Install scons
         run: pip install scons
+      - name: Set locale
+        run: sudo locale-gen en_US.UTF-8 && echo $LC_ALL
       - name: Install gettext
         run: sudo apt-get install gettext
       - name: Generating pot file