fix: move env block to job-level in windows.yml workflow

[Rayhan1967]

Summary

• Fix invalid YAML syntax in .github/workflows/windows.yml where env block was placed before the on trigger
• Moved environment variables (OPAMROOT, OPAMSOLVERTIMEOUT, MSYS) to job-level for both analyse and build jobs
• This follows correct GitHub Actions workflow syntax as per documentation

Changes

Before:

name: Windows CI
on:
  pull_request:
env:          # <-- Invalid: placed before 'on'
  OPAMROOT: D:\opamroot
jobs:
  ...

After:

name: Windows CI
on:
  pull_request:
jobs:
  analyse:
    runs-on: ubuntu-latest
    env:    # <-- Correct: job-level env
      OPAMROOT: D:\opamroot
    ...
  build:
    runs-on: windows-latest
    env:    # <-- Correct: job-level env
      OPAMROOT: D:\opamroot
    ...

Testing

• YAML syntax validated successfully

[jmid]

Those env lines were deleted as part of #29614 which moved the GHA workflows to setup-ocaml.
(this is also the reason for the reported merge conflicts)

As such, this PR is unfortunately no longer relevant.