Disable GitHub certificate check under Ubuntu 18.04

wget doesn't recognize it for some reason under this old OS, so just
disable it to be able to download mold there.

.github/workflows/ci.yml

@@ -104,6 +104,14 @@ jobs:
               ;;
           esac
 
+          case "${{matrix.container}}" in
+            ubuntu:18.04)
+              # We need to use this option as GitHub certificate is not recognized by
+              # wget in this old container otherwise.
+              set_env_var SOCI_WGET_OPTIONS --no-check-certificate
+              ;;
+          esac
+
           if [ -n "${{matrix.cxxstd}}" ]; then
             set_env_var SOCI_CXXSTD ${{matrix.cxxstd}}
           fi

scripts/ci/install.sh

@@ -44,7 +44,7 @@ case "$(uname)" in
         # Get mold and replace the default linker with it.
         mold_ver=2.4.0
         mold_name=mold-${mold_ver}-$(uname -m)-linux.tar.gz
-        wget https://github.com/rui314/mold/releases/download/v${mold_ver}/${mold_name}
+        wget $SOCI_WGET_OPTIONS https://github.com/rui314/mold/releases/download/v${mold_ver}/${mold_name}
         sudo tar -C /usr/local --strip-components=1 -xzf ${mold_name}
         sudo ln -sf /usr/local/bin/mold /usr/bin/ld
         ;;