GithubActionsCI(snap): switch from 18.04 to 20.04

The reason is twofold:
- Ubuntu 20.04 is finally available according to [1].
- Builds with Ubuntu 18.04 have started failing with this
error ([2], [3]):
```
Failed to update the package cache: Some files could not be downloaded:

W:GPG error: https://storage.googleapis.com/bazel-apt stable InRelease: The following signatures were invalid: EXPKEYSIG 3D5919B448457EE0 Bazel Developer (Bazel APT repository key) <[email protected]>
E:The repository 'https://storage.googleapis.com/bazel-apt stable InRelease' is not signed.
```

This is essentially trying again [4] (reverting [5]) but in the
master branch.

[1] actions/runner-images#228
[2] https://github.com/nblockchain/geewallet/runs/735263044
[3] https://github.com/nblockchain/geewallet/runs/735211396
[4] de59e12
[5] 2ce094f

.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

snap/snapcraft.yaml

@@ -1,5 +1,5 @@
 name: gwallet
-base: core18 # the base snap is the execution environment for this snap
+base: core20 # the base snap is the execution environment for this snap
 version: '0.3.209.0' # just for humans, typically '1.2+git' or '1.3.2'
 summary: minimalistic cryptocurrency brainwallet # 79 char long summary
 description: |