Switch 5.1.0 workflows to rc1

[jmid]

This PR switches the GitHub action workflows to test 5.1.0~rc1.

I've created a PR against shym/custom-opam-repository to switch under Mingw and Cygwin shym/custom-opam-repository#2. Until that is merged and in the interest of testing 5.1.0~rc1 the most, the PR is temporarily using the relevant branch from jmid/custom-opam-repository

[jmid]

CI summary:

• Cygwin 5.1 part 1 timeout due to general slowness (2299.8s Domain.spawn/join - atomic, 2834.6s Lin DSL Bigarray.Array1 (of ints) test with Domain, 2358.4s Lin DSL Bytes test with Domain, ...)
• Cygwin trunk/5.2 part 2 timeout during a 10933.3s Lin ref int64 test with Thread shrink run Improve reproducibility of Lin/STM Thread modes #338
• Linux trunk debug timeout during 11346.8s Lin DSL In_channel test with Domain Long In_channel shrink runs causing timeout #373 (a 2 days old CI run, should be fixed by In_channel: Generate and shrink bytes only on length #375 merged yesterday)
• Mingw bytecode 5.1 and Mingw bytecode trunk timeouts during threadomain [ocaml5-issue] Windows failures on threadomain #203
• Mingw trunk crash of threadomain [ocaml5-issue] Windows failures on threadomain #203
• ppc64 trunk/5.2 triggered an infinite loop on STM Array test sequential [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380

(+ 2 ongoing Cygwin part 2s)

[jmid]

The last Cygwin runs finished, so I've updated the above summary. None of these issues are new, so I'll merged.