fix(bootstrap): retry transient Fill failures when resolving build jars

A single HTTP 502 from fill.papermc.io aborted the entire bootstrap. Build
resolution used a one-shot curl, so one gateway blip from an upstream that
flaps was indistinguishable from a permanent failure, and the run died before
Docker, k3s or any game server was provisioned.

Pass --retry 5 --retry-delay 2 to the build-resolution fetches. 502/503/504
are already in curl's built-in transient set, so the tool had solved this; the
flags were simply never passed. papermc_latest_jar is shared by the Paper and
the Velocity resolve, so hardening it once covers both callers. The
LOOHP/Limbo CI metadata fetch feeds the same step and gets the same treatment.

Deliberately no --retry-connrefused. It only adds ECONNREFUSED to a set that
already covers this incident, and it needs curl 7.52.0 while the yum (el7)
path the script supports ships 7.29.0, where an unrecognised long option is a
parse error rather than a warning:

  # centos:7, curl 7.29.0
  $ curl -fsSL --retry 5 --retry-delay 2 --retry-connrefused https://example.com
  curl: option --retry-connrefused: is unknown

Under set -Eeuo pipefail that exits 2 and trips the || die, so both hardened
fetches would hard-fail on a host where they used to work, each naming a cause
that is not the real one. A comment above papermc_latest_jar records this so
the flag does not come back.

The failure message was actively misleading. "no Paper build for Minecraft
26.2 (the login gate speaks only that protocol)" reads as "that Minecraft
version is unsupported", sending the reader after a version-pinning problem
that does not exist: Paper 26.2 build 60 resolved fine minutes later. Say what
is actually known instead, that the build likely exists and Fill is flapping.

Verified against a local always-502 server: curl now issues 6 requests
(1 initial + 5 retries) over 10.1s before giving up, where it previously
issued 1 and died. Verified on centos:7 that this flag set is accepted, and
against the live Fill v3 API that the resolve still returns a jar URL.

Known and deliberately unchanged: no fetch sets --max-time, so an upstream
that accepts a connection and never answers still blocks forever. --retry does
not cover that, as it fires only once a request completes with a failure. The
remaining single-shot downloads (cloudflared, the Docker GPG key and repo
list, k3s, the Temurin JRE, the Velocity jar, the Go toolchain) keep their
existing no-retry shape rather than widen this diff on a script that is about
to provision a live host.