chore: update CI to OCaml 5.5

[Alizter]

Description

Update the OCaml version used by CI and development tooling from 5.4 to 5.5.

This updates the opam CI matrix, benchmark workflow, local development switch,
tutorial, and Nix development shell. The flake inputs are refreshed for the
matching Melange 5.5 stack and include the MinGW OCaml 5.5 fixes needed by the
static Windows package set.

Package-set consistency

nix-overlays selects a post-0.38.0 ppxlib commit that no longer depends on
stdlib-shims, while opam installs the official ppxlib 0.38.0 release that
still declares this dependency. This made dune describe report different
dependency graphs in the two CI environments.

The Nix package set now uses nixpkgs ppxlib 0.38.0 release source, matching
opam and keeping the stdlib-shims dependency graph consistent across package
managers.

OCaml 5.5 test compatibility

OCaml 5.5 relocatable compiler lookup fails in tests that construct a restricted
PATH from symlinked compiler tools. Those tests now capture the real standard
library directory in OCAMLLIB before replacing PATH.

Findlib 1.9.8 also warns about the relative stub-library entries in OCaml 5.5
ld.conf. Tests that invoke findlib without exercising stub lookup set
OCAMLFIND_LDCONF=ignore, preventing those unrelated warnings from changing
cram output.

Related Issue and Motivation

The OCaml 5 release tested by CI and used by local development should track the
current release.

Fixes #15445

Validation

• ./dune.exe build @check @fmt @runtest
• Affected cram tests under the OCaml 5.5 Nix shell
• Nix ppxlib and its reverse-dependency closure
• git diff --check

Checklist

• Existing tests cover the affected behavior.
• No changelog entry is required for CI and test-environment changes.
• The development tutorial uses OCaml 5.5.

[shonfeder]

Thanks!

[Alizter]

Current CI failure checklist:

• Make dune describe expectations consistent across Nix and opam OCaml 5.5. Nix now uses the same ppxlib 0.38.0 release source as opam, so both dependency closures include stdlib-shims.

• Preserve the real OCaml standard-library path in tests that fake PATH while hiding tools such as melc. The affected tests now set OCAMLLIB before constructing the restricted path.

• Remove unrelated ocamlfind warning noise about relative stublibs entries. Tests that do not exercise stub lookup now set OCAMLFIND_LDCONF=ignore.