Cirrus-CI: retry pkg installation on failure

Pkg installation failed somewhat frequently, always at:

[62/104] Fetching jpeg-turbo-2.0.6.txz: .......... done
pkg: http://pkgmir.geo.freebsd.org/FreeBSD:13:amd64/quarterly/All/jbigkit-2.1_1.txz: No route to host

Move pkg installation to a script and retry once upon failure as a
(hopefully temporary) workaround.

Reviewed by:	imp
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30613

.cirrus-ci/pkg-install.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+set -e
+
+pkg install -y "$@" && exit 0
+
+cat <<EOF
+pkg install failed
+
+dmesg tail:
+$(dmesg | tail)
+
+trying again
+EOF
+
+pkg install -y "$@"

.cirrus.yml

@@ -17,7 +17,7 @@ task:
   name: World and kernel amd64 build and boot smoke test
   timeout_in: 120m
   install_script:
-  - pkg install -y qemu uefi-edk2-qemu-x86_64 llvm12
+  - sh .cirrus-ci/pkg-install.sh qemu uefi-edk2-qemu-x86_64 llvm12
   setup_script:
   - uname -a
   - df -m