GHA: drop heisensegfaulting ubuntu-latest envs

Author: Florian Mayer

.github/workflows/main.yml

@@ -33,8 +33,8 @@ jobs:
           # - { os: ubuntu-20.04, r: "release"} # not released yet
           # - { os: ubuntu-20.04, r: "oldrel" } # not released yet
           # - { os: ubuntu-latest, r: "devel" } # fails setup-r
-          - { os: ubuntu-latest, r: "release"} # ubuntu-18.04
-          - { os: ubuntu-latest, r: "oldrel" }
+          #- { os: ubuntu-latest, r: "release"} # ubuntu-18.04 # heisensegfault
+          #- { os: ubuntu-latest, r: "oldrel" } # heisensegfault
           # - { os: ubuntu-16.04, r: "devel" } # fails setup-r
           - { os: ubuntu-16.04, r: "release"}
           - { os: ubuntu-16.04, r: "oldrel" }