cirrus: Switch to big-sur image, use catalina for bootstrap only

.cirrus.yml

@@ -83,9 +83,9 @@ task:
 
 # Mac
 task:
-  name: macOS 10.15 x64, $TASK_NAME_SUFFIX
+  name: macOS 11.x x64, $TASK_NAME_SUFFIX
   osx_instance:
-    image: catalina-xcode
+    image: big-sur-xcode
   timeout_in: 60m
   environment:
     OS_NAME: darwin
@@ -97,7 +97,19 @@ task:
       - TASK_NAME_SUFFIX: DMD (latest)
       - TASK_NAME_SUFFIX: DMD (coverage)
         << : *COVERAGE_ENVIRONMENT_TEMPLATE
-      - TASK_NAME_SUFFIX: DMD (bootstrap)
+  << : *COMMON_STEPS_TEMPLATE
+
+task:
+  name: macOS 10.15 x64, DMD (bootstrap)
+  osx_instance:
+    image: big-sur-xcode
+  timeout_in: 60m
+  environment:
+    OS_NAME: darwin
+    # override Cirrus default OS (`darwin`)
+    OS: osx
+    # 12 CPU cores and 24 GB of memory are available
+    N: 12
     # de-facto bootstrap version on OSX
     # See: https://forum.dlang.org/post/qfsgt2$1goc$1@digitalmars.com
     HOST_DMD: dmd-2.088.0

cirrusci.sh

@@ -30,7 +30,7 @@ if [ "$OS_NAME" == "linux" ]; then
 elif [ "$OS_NAME" == "darwin" ]; then
   # required for dlang install.sh
   brew update-reset
-  brew install gnupg
+  brew install gnupg libarchive xz
 elif [ "$OS_NAME" == "freebsd" ]; then
   packages="git gmake"
   if [ "$HOST_DMD" == "dmd-2.079.0" ] ; then