CI: setup-ocaml 3.4.6 / OPAM 2.4 / windows-2025 runners

[cknitt]

• Upgrade setup-ocaml to 3.4.6 which enables OPAM 2.4/2.5
• Use windows-2025 runners
• Make cache keys dependent on setup-ocaml version
• Clean up env vars

[pkg-pr-new]

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rescript

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commit: 323c24d