Remove installed packages from Mirage duniverse

[funwithcthulhu]

Summary

• In the Mirage example CI job, remove duniverse directories whose package names are already installed in the opam switch.
• Force plain opam list output so CI color settings do not leak escape codes into rm arguments.
• Keep explicit removals for duniverse directory names that do not match opam package names, such as ocaml-cstruct, mirage-tcpip, and bytes.
• Build only the generated Mirage ./main.exe target instead of the default alias, so vendored tests and examples are not part of this smoke test.

This addresses the duplicate-library failures currently seen in Mirage CI, including ohex, lwt-dllist, logs, tcpip, and bytes.

Validation

• git diff --check
• Observed failing Mirage logs on Document decoded route literals #423/Allow customizing the logger middleware source #424/Remove installed packages from Mirage duniverse #426 showing duplicate libraries from both duniverse and _opam/lib, plus unrelated vendored test/example aliases being built by the default target.