switch CI to the supported versions of ubuntu and macos (#33)

* switch CI to the supported versions of ubuntu and macos

See:
- https://github.com/actions/runner-images/issues/6002
- https://github.com/actions/runner-images/issues/5583

* fix for `nnkArgList` style

.github/workflows/ci.yml

@@ -26,11 +26,11 @@ jobs:
         include:
           - target:
               os: linux
-            builder: ubuntu-18.04
+            builder: ubuntu-20.04
             shell: bash
           - target:
               os: macos
-            builder: macos-10.15
+            builder: macos-11
             shell: bash
           - target:
               os: windows

faststreams.nimble

@@ -18,7 +18,16 @@ proc test(args, path: string) =
   # Compilation language is controlled by TEST_LANG
   let lang = getEnv("TEST_LANG", "c")
 
-  var common_args = "-r -f " & getEnv("NIMFLAGS") &  " --hints:off --styleCheck:usages --styleCheck:error"
+  # nnkArglist was changed to nnkArgList, so can't always use --styleCheck:error
+  # https://github.com/nim-lang/Nim/pull/17529
+  # https://github.com/nim-lang/Nim/pull/19822
+  let styleCheckStyle =
+    if (NimMajor, NimMinor) < (1, 6):
+      "hint"
+    else:
+      "error"
+
+  var common_args = "-r -f " & getEnv("NIMFLAGS") &  " --hints:off --styleCheck:usages --styleCheck:" & styleCheckStyle
 
   if getEnv("NIMBUS_ENV_DIR") != "":
     common_args &= " --skipParentCfg"