Fix `sed` on MacOS and remove --pin and updates on CI

This MR implements 5 changes:

• Fix usage of sed on MacOS in the script tools/init_opam_switch.sh. It is necessary to add the flag -E to support extended regular expressions and to use pipes without escape. This MR is adding variables to consider this depending on the operating system. Another solution would be to restrict ourselves to basic regular expressions in sed commands, which might keep the script more readable.
• Finish removing support for --pin. This feature exists on the main branch but seems to be disabled on dev. This MR is removing one last snippet that is related to it.
• Add conditions for running the Windows pipeline on CI. It will only run if one of the following conditions are met:
  • The name of the branch receiving a push ends with for-windows-ci.
  • The push is on a branch with an MR open either to dev or to main.
• Remove _nojs from CI for 5.0.0.
• Add CI pipeline for OCaml 5.1.0. We are using _nojs here since compiling cduce-js with 5.1.0 results in the following warning:

There are some missing primitives
Dummy implementations (raising 'Failure' exception) will be used if they are not available at runtime.
You can prevent the generation of dummy implementations with the commandline option '--disable genprim'
Missing primitives:
  length_nat