fix(install): don't require the GitHub API when RAILWAY_VERSION is pinned

[krushiraj]

Fixes #1030.

Problem

install.sh queries the GitHub API for the latest release at line 411 — unconditionally, and with a hard exit 1 on an empty result — before it checks whether the caller already pinned RAILWAY_VERSION (line 420), and before argument parsing (line 451).

DEFAULT_VERSION=$(curl -s https://api.github.com/repos/railwayapp/cli/releases/latest | grep -o ...)

if [ -z "$DEFAULT_VERSION" ]; then
  error "Failed to fetch latest version from GitHub"
  exit 1
fi

if [ -z "${RAILWAY_VERSION-}" ]; then
  RAILWAY_VERSION="$DEFAULT_VERSION"
fi

Three consequences when the API is rate-limited or unreachable:

1. A pinned install fails. RAILWAY_VERSION=5.27.1 bash <(curl -fsSL railway.com/install.sh) errors out with Failed to fetch latest version from GitHub, even though DEFAULT_VERSION is discarded on the very next line. This is the reported failure — it bites CI pipelines that pin a version specifically to be insulated from rate limits.
2. --help and --uninstall fail too. Both are handled well after line 411, so removing an already-installed binary requires a working network path to api.github.com.
3. It always surfaces as this error rather than a curl failure: curl -s (no -f) exits 0 on a 403 and prints the rate-limit JSON body, so set -eu doesn't catch it — grep just finds no tag_name and DEFAULT_VERSION ends up empty.

Fix

Move the lookup into resolve_version(), return early when RAILWAY_VERSION is already set, and call it after argument parsing and after the uninstall/help blocks — immediately before the first use of $RAILWAY_VERSION. DEFAULT_VERSION had no other readers and is dropped. The unpinned failure path also now hints at pinning.

Verification

bash -n passes. Stubbing curl to return GitHub's 403 rate-limit body, on master vs. this branch:

Scenario	master	This PR
RAILWAY_VERSION=5.27.1	Failed to fetch latest version	resolves .../download/v5.27.1/... ✅
--help	Failed to fetch latest version	prints help ✅
--uninstall	Failed to fetch latest version	uninstalls ✅
no pin (regression check)	error	same error, plus a pinning hint ✅

Shell-only change; no Rust code is touched.