Cirrus CI: Extract prerequisites installation to cirrusci.sh

As preparation for druntime/Phobos CI, so that they can use this file
too instead of having to duplicate these steps in their .cirrus.yml.

.cirrus.yml

@@ -1,40 +1,16 @@
 common_steps_template: &COMMON_STEPS_TEMPLATE
-  install_prerequisites_script: |
-    set -uexo pipefail
-    if [ "$OS_NAME" == "linux" ]; then
-      packages="git-core make g++ gdb curl libcurl3 tzdata zip unzip xz-utils"
-      if [ "$MODEL" == "32" ]; then
-        dpkg --add-architecture i386
-        packages="$packages g++-multilib libcurl3-gnutls:i386"
-      fi
-      if [ "${DMD:0:4}" == "gdmd" ]; then
-        packages="$packages sudo software-properties-common wget"
-      fi
-      apt-get -q update
-      apt-get install -yq $packages
-    elif [ "$OS_NAME" == "darwin" ]; then
-      # required for install.sh
-      brew install gnupg
-    elif [ "$OS_NAME" == "freebsd" ]; then
-      packages="git gmake bash"
-      if [ "${D_VERSION:-x}" == "2.079.0" ] ; then
-        packages="$packages lang/gcc9"
-      fi
-      pkg install -y $packages
-      rm /usr/bin/make
-      ln -s /usr/local/bin/gmake /usr/bin/make
-    fi
-    # create a `dmd` symlink to the repo dir, necessary for druntime/Phobos
-    ln -s $CIRRUS_WORKING_DIR ../dmd
+  install_prerequisites_script: ./cirrusci.sh
   install_host_compiler_script: |
+    set -uexo pipefail
     # kludge for ci.sh
     if [ "${DMD:0:4}" == "gdmd" ]; then export DMD="gdmd"; fi
     compiler="$DMD"
     if [ -n "${D_VERSION+x}" ]; then compiler="$DMD-$D_VERSION"; fi
     ./ci.sh install_d "$compiler"
   setup_repos_script: |
-    export BRANCH=${CIRRUS_BASE_BRANCH:-$CIRRUS_BRANCH}
-    ./ci.sh setup_repos
+    set -uexo pipefail
+    ln -s $CIRRUS_WORKING_DIR ../dmd
+    BRANCH="${CIRRUS_BASE_BRANCH:-$CIRRUS_BRANCH}" ./ci.sh setup_repos
   build_script: ./ci.sh build
   test_dmd_script: ./ci.sh test_dmd
   test_druntime_script: ./ci.sh test_druntime
@@ -46,7 +22,6 @@ environment:
   MODEL: 64
   DMD: dmd
   N: 4
-  BRANCH: master
   OS_NAME: linux
   FULL_BUILD: true
 
@@ -103,6 +78,7 @@ task:
   timeout_in: 60m
   environment:
     OS_NAME: freebsd
+  install_bash_script: pkg install -y bash
   << : *COMMON_STEPS_TEMPLATE
 
 task:
@@ -115,4 +91,5 @@ task:
   environment:
     OS_NAME: freebsd
     D_VERSION: 2.079.0
+  install_bash_script: pkg install -y bash
   << : *COMMON_STEPS_TEMPLATE