snap: try building&releasing from 20.04 instead of 18.04

This commit will try building a 0.4.207.0 version (marked as 'beta'
in the snap store) with this new distro version, to see if it works
and if the default GTK theme matches the one supplied by the distro.

.github/workflows/ubuntu.yml

@@ -1,13 +1,10 @@
 name: ubuntu
 
-on:
-  push:
-    tags:
-    - '*'
+on: [push]
 
 jobs:
   ubuntu-snap-build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v1
     - name: Generate snap package
@@ -24,7 +21,7 @@ jobs:
 
   ubuntu-snap-upload:
     needs: ubuntu-snap-build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v1
     - name: Download artifact from previous job

scripts/snap_release.sh

@@ -19,4 +19,4 @@ fi
 
 snapcraft login --with snapcraft.login
 
-snapcraft push snap/*.snap --release=stable
+snapcraft push snap/*.snap --release=beta