use ubuntu 18.04

.github/workflows/main.yml

@@ -7,7 +7,7 @@ on: [push, pull_request]
 jobs:
   cancel:
     name: "Cancel previous runs"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     timeout-minutes: 3
     steps:
       - uses: styfle/[email protected]
@@ -232,7 +232,7 @@ jobs:
       
 
   linux-build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       PLATFORM: linux64
       OPAMYES: 1
@@ -272,6 +272,7 @@ jobs:
       - name: Install dependencies
         run: |
           set -ex
+          sudo add-apt-repository ppa:avsm/ppa -y # provides OPAM 2
           sudo add-apt-repository ppa:haxe/ocaml -y # provides newer version of mbedtls
           sudo apt-get update -qqy
           sudo apt-get install -qqy ocaml-nox camlp5 opam libpcre3-dev zlib1g-dev libgtk2.0-dev libmbedtls-dev ninja-build libstring-shellquote-perl
@@ -639,7 +640,7 @@ jobs:
 
   linux-test:
     needs: linux-build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       PLATFORM: linux64
       TEST: ${{matrix.target}}
@@ -904,7 +905,7 @@ jobs:
   deploy_apidoc:
     if: github.event_name != 'pull_request' # TODO: also only when `GHP_REMOTE` is present
     needs: [linux-test, mac-test, windows-test, windows64-test]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: Download Haxe
         uses: actions/download-artifact@v2

extra/github-actions/build-linux.yml

@@ -1,6 +1,7 @@
 - name: Install dependencies
   run: |
     set -ex
+    sudo add-apt-repository ppa:avsm/ppa -y # provides OPAM 2
     sudo add-apt-repository ppa:haxe/ocaml -y # provides newer version of mbedtls
     sudo apt-get update -qqy
     sudo apt-get install -qqy ocaml-nox camlp5 opam libpcre3-dev zlib1g-dev libgtk2.0-dev libmbedtls-dev ninja-build libstring-shellquote-perl

extra/github-actions/workflows/main.yml

@@ -6,7 +6,7 @@ on: [push, pull_request]
 jobs:
   cancel:
     name: "Cancel previous runs"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     timeout-minutes: 3
     steps:
       - uses: styfle/[email protected]
@@ -53,7 +53,7 @@ jobs:
       @import build-windows.yml
 
   linux-build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       PLATFORM: linux64
       OPAMYES: 1
@@ -132,7 +132,7 @@ jobs:
 
   linux-test:
     needs: linux-build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       PLATFORM: linux64
       TEST: ${{matrix.target}}
@@ -265,7 +265,7 @@ jobs:
   deploy_apidoc:
     if: github.event_name != 'pull_request' # TODO: also only when `GHP_REMOTE` is present
     needs: [linux-test, mac-test, windows-test, windows64-test]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: Download Haxe
         uses: actions/download-artifact@v2