ci: load generated environment before install doctor checks

[naokihaba]

#2758 introduced a separate fallback shim directory for system-first tools, so vp env doctor now expects both the main shim directory and the fallback one in PATH.

However, the standalone install workflow only added ~/.vite-plus/bin to GITHUB_PATH. Since Git Bash on Windows doesn't inherit the persistent user PATH set by the installer, the fallback directory was missing when running vp env doctor.

This didn't show up in #2758 because the workflow installs the published CLI, which didn't have the new check yet. It started failing once v1.0.0-rc.0 was published with those changes.

I was able to reproduce the failure on main. This PR fixes it by sourcing ~/.vite-plus/env before running vp env doctor, which mirrors the actual user setup where the main directory comes first in PATH and the fallback directory comes last.

https://github.com/voidzero-dev/vite-plus/actions/runs/35752079139/job/106828512682?pr=2785

Found trampoline shim: /c/Users/runneradmin/.vite-plus/bin/vp.exe
Installation
  ✓ Bin dir           ~\.vite-plus\bin
  ✓ Data dir          ~\.vite-plus
  ✓ Cache dir         ~\.vite-plus\cache
  ✓ Config dir        ~\.vite-plus
  ✓ State dir         ~\.vite-plus
  ✓ Shims             node, npm, npx, pnpm, pnpx, pn, pnx, yarn, yarnpkg, bun, bunx, vpx, vpr

Configuration
  ✓ Node.js           managed mode
  ✓ Package manager   managed mode

PATH
  ✓ vp                ~\.vite-plus\bin\vp.exe
  ✓ Shim dir          ~\.vite-plus\bin
  ✗ Fallback dir      not in PATH
  Add the bin directory to your PATH via:
  System Properties -> Environment Variables -> Path

  Then restart your terminal.

Node.js Resolution
    Directory         D:\a\vite-plus\vite-plus
    Source            D:\a\vite-plus\vite-plus\.node-version
    Version           22.18.0
  ⚠ Node binary       not installed
  note: Version will be downloaded on first use.

Package Manager Resolution
    Source            packageManager
    Version           pnpm@11.24.0
  ✓ PM binaries       installed

✗ Some issues found. Run the suggested commands to fix them.
Error: Process completed with exit code 1.