[actions] retry apt package installs through Ubuntu mirror outages

Mirror outages have outlasted the existing retries,
which covered only `apt-get update` (and not the WSL `additional-packages` install).
Installs now go through `.github/scripts/apt-install.sh`,
which retries each request within apt and retries the whole update+install with backoff for about 10 minutes.

.github/scripts/apt-install.sh

@@ -0,0 +1,49 @@
+#!/bin/sh
+
+# Install apt packages, riding out Ubuntu/Debian mirror outages: apt itself
+# retries each request, and the whole update+install is retried with backoff
+# for about 10 minutes, since outages have lasted longer than apt's own retries.
+#
+# usage: sh .github/scripts/apt-install.sh <package>...
+# Runs apt-get through sudo when not root.
+
+set -u
+
+if [ "$#" -eq 0 ]; then
+  echo 'usage: apt-install.sh <package>...' >&2
+  exit 2
+fi
+
+SUDO=''
+if [ "$(id -u)" != '0' ]; then
+  SUDO='sudo'
+fi
+
+apt_get() {
+  $SUDO env DEBIAN_FRONTEND=noninteractive apt-get \
+    -o Acquire::Retries=5 \
+    -o Acquire::http::Timeout=30 \
+    -o Acquire::https::Timeout=30 \
+    "$@"
+}
+
+ATTEMPT=1
+MAX_ATTEMPTS=8
+DELAY=15
+while :; do
+  apt_get update || echo "apt-get update failed (attempt ${ATTEMPT}/${MAX_ATTEMPTS})" >&2
+  if apt_get install -y "$@"; then
+    exit 0
+  fi
+  if [ "${ATTEMPT}" -ge "${MAX_ATTEMPTS}" ]; then
+    echo "apt-get install failed after ${MAX_ATTEMPTS} attempts: $*" >&2
+    exit 1
+  fi
+  echo "apt-get install failed (attempt ${ATTEMPT}/${MAX_ATTEMPTS}); retrying in ${DELAY}s" >&2
+  sleep "${DELAY}"
+  ATTEMPT=$((ATTEMPT + 1))
+  DELAY=$((DELAY * 2))
+  if [ "${DELAY}" -gt 120 ]; then
+    DELAY=120
+  fi
+done