Move GitHub actions to test 5.1.0~beta1

[jmid]

This PR moves the GitHub actions CI to test the fresh 5.1.0~beta1 release, by building on a custom Windows opam package as we've done for 5.1.0~alpha2 shym/custom-opam-repository#1

[shym]

LGTM!

[jmid]

CI summary:

• 32-bit trunk and linux-s390x-5.1 unexpectedly triggered a Lin CList int test with Thread counterexample Lin.Internal Thread int CList test finds unexpected counterexample under bytecode #358
• both macOS 5.1 and trunk failed to trigger STM int64 ref test parallel asymmetric 'STM _ ref test parallel asymmetric' failure to trigger #364
• linux-arm64-5.1 and linux-ppc64le-5.2 failed to trigger Lin DSL Out_channel test with Domain Reliability of parallel Lin Out_channel tests #360
• Cygwin trunk part1 timed out due to general slowness (1181.9s Domain.spawn/join - atomic, 2995.9s domain_spawntree - with Atomic, 1356.7s Lin DSL Atomic test with Domain, 1824.0s Lin DSL Bytes test with Domain
• macos-arm64-5.1 failed with an Internal error

So a few things to look into, but nothing caused by this PR. I'll therefore merge.