switch CI to the supported version of ubuntu (#139)

* switch CI to the supported version of ubuntu

See https://github.com/actions/runner-images/issues/6002

* don't continue on error for Nim 1.6

* install openssl on macos for Nim devel

due to changes in https://github.com/nim-lang/Nim/pull/19814

---------

Co-authored-by: Jacek Sieka <jacek@status.im>

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
         include:
           - target:
               os: linux
-            builder: ubuntu-18.04
+            builder: ubuntu-20.04
             shell: bash
           - target:
               os: macos
@@ -72,6 +72,13 @@ jobs:
           chmod 755 external/bin/gcc external/bin/g++
           echo '${{ github.workspace }}/external/bin' >> $GITHUB_PATH
 
+      - name: 'Install dependencies (macOS)'
+        if: runner.os == 'macOS' && matrix.branch == 'devel'
+        run: |
+          brew install openssl@1.1
+          ln -s $(brew --prefix)/opt/openssl/lib/libcrypto.1.1.dylib /usr/local/lib
+          ln -s $(brew --prefix)/opt/openssl/lib/libssl.1.1.dylib /usr/local/lib/
+
       - name: MSYS2 (Windows i386)
         if: runner.os == 'Windows' && matrix.target.cpu == 'i386'
         uses: msys2/setup-msys2@v2