Actions update

[jmid]

This little PR updates a couple of our GitHub Actions to the latest releases:

• actions/checkout@v4
• cygwin/cygwin-install-action@v4

I don't expect any immediate improvement by doing so, but we may benefit from future ones by doing so.

[jmid]

CI summary for 7cd616d

• Linux 5.1 debug triggered an assertion failure in thread_joingraph [ocaml5-issue] Abort / crash on thread_joingraph and thread_createtree under debug runtime #353
• Linux trunk debug triggered an assertion failure in both threadomain [ocaml5-issue] Windows failures on threadomain #203 and in thread_createtree
• Cygwin 5.1 / part2 timeout during a 5547.8s Lin.Internal CList int64 test with Thread Timeouts during 'Lin.Internal CList int64 test with Thread' under Cygwin #397

Out of 63 workflows 3 failed - with 2 genuine failures and one borderline false alarm

[jmid]

CI summary for c5412c4:

• 32bit 5.1 found an unexpected counterexample in Lin Queue test with Thread Lin Thread Queue test reliability #363
• Cygwin 5.1 / part1 timeout during negative Lin Dynlink test with Domain (shrinking) [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• Linux trunk debug timeout during 12214.9s Lin Out_channel test with Domain shrink - also triggered a threadomain assertion failure Long Out_channel shrinking runs cause timeout #378 [ocaml5-issue] Assertion failure on threadomain under debug runtime #404
• MinGW 5.1 timeout during threadomain [ocaml5-issue] Windows failures on threadomain #203
• MinGW bytecode 5.1 found an unexpected counterexample in Lin Queue test with Thread Lin Thread Queue test reliability #363
• MinGW bytecode trunk timeout during threadomain [ocaml5-issue] Windows failures on threadomain #203
• MinGW bytecode trunk found an unexpected counterexample in Lin Queue test with Thread Lin Thread Queue test reliability #363
• freebsd-amd64-5.0 failed to find a counterexample in STM Sys test parallel FreeBSD failure to trigger 'STM Sys test parallel' #410

Out of 63 runs 8 failed with 4 genuine and 5 false alarms (one run had both)

[jmid]

CI summary for merge to main:

• MinGW trunk failed for an unknown reason as the relevant part of the actions log is unavailable! https://github.com/ocaml-multicore/multicoretests/actions/runs/6816588892/job/18538228573
• freebsd-amd64-5.1 completed Lin Out_channel test with Domain without finding a counterexample Lin Out_channel test fails to trigger on FreeBSD #401

Out of 38 workflows 2 failed - 1 false alarm and 1 unknown