Refresh the package lists before installing in script/bootstrap (closes #508)

The CI runner's image starts with empty package lists, so `script/bootstrap`, which `script/cibuild` now runs first, failed to install Go and every CI run stopped there. The apt branch of `pkg_install` now runs `apt-get update` once per run, before its first `apt-get install`, with the same code as the pending shared fix in sneak/prompts#116.

Deviation: `script/bootstrap` differs from the vendored copy until the next re-vendor.
Unverified: proven by running the CI job in the runner's image; the branch's own CI run waits for the runner outage to clear.

Model: opus-5-5

script/bootstrap

@@ -11,6 +11,7 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 PKGMGR=""
 SUDO=""
+APT_UPDATED=""
 
 detect_pkgmgr() {
     [ -n "$PKGMGR" ] && return 0
@@ -39,7 +40,14 @@ pkg_install() {
     detect_pkgmgr
     case "$PKGMGR" in
         nix) nix-env -iA "nixpkgs.$1" ;;
-        apt) $SUDO env DEBIAN_FRONTEND=noninteractive apt-get install -y "$2" ;;
+        apt)
+            # Package lists may be empty (fresh images); refresh once per run.
+            if [ -z "$APT_UPDATED" ]; then
+                $SUDO env DEBIAN_FRONTEND=noninteractive apt-get update
+                APT_UPDATED=1
+            fi
+            $SUDO env DEBIAN_FRONTEND=noninteractive apt-get install -y "$2"
+            ;;
         brew) brew install "$3" ;;
         apk) apk add --no-cache "$4" ;;
     esac