Refresh apt package lists before bootstrap installs anything (closes #12)

The CI job image ships with empty apt package lists, so the bootstrap's
`apt-get install -y golang` could not find the package and every CI run
stopped there. On the apt path the bootstrap now runs `apt-get update`
once, before the first install, and not at all when nothing needs
installing.

Model: opus-5-5

script/bootstrap

@@ -31,6 +31,9 @@ detect_pkgmgr() {
 		if [ "$(id -u)" != "0" ]; then
 			SUDO="sudo"
 		fi
+		# Fresh images, the CI runner's among them, ship with empty
+		# package lists. This runs once, on the first install.
+		$SUDO env DEBIAN_FRONTEND=noninteractive apt-get update
 	fi
 }