force ubuntu-18.04 for github actions

building on ubuntu 20.04 was causing issue with a too recent glibc for
some users dependent on ubuntu 18.04 or similar (elementary OS)
The issue arise as github action ubuntu-latest was upgraded recently too
ubuntu 20.04.
This fix forces the app to be built on ubuntu 18.04

Relates #1471

.github/workflows/build-binaries.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-2016, macos-latest, ubuntu-latest]
+        os: [windows-2016, macos-latest, ubuntu-18.04]
     env:
       SIGNAL_ENV: production
       GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/pull-request.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-2016, macos-latest, ubuntu-latest]
+        os: [windows-2016, macos-latest, ubuntu-18.04]
     env:
       SIGNAL_ENV: production
       GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/release.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-2016, macos-latest, ubuntu-latest]
+        os: [windows-2016, macos-latest, ubuntu-18.04]
     env:
       SIGNAL_ENV: production
       GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}