Switch 5.1.0 GitHub action workflows to 5.1.0~rc3

[jmid]

This PR switches the 5.1.0 GitHub action workflows to 5.1.0~rc3.
The most exiting part is spotting that .github/workflows/linux-510-fp.yml was using 5.1.0~rc1... 😅
Almost nothing to see here, move along!

[jmid]

CI summary:

• Linux frame pointer 5.1 segfault failure [ocaml5-issue] Segfault with frame pointers in Effect test #394
• Linux ppc64le 5.2 failed with SEGV on STM Bytes test sequential [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380
• Cygwin trunk part 1 timeout during the Lin Dynlink test [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• macOS 5.1 failed to trigger STM int64 ref test parallel asymmetric in 5000 iterations 'STM _ ref test parallel asymmetric' failure to trigger #364
• macOS trunk failed to trigger STM int and int64 ref test parallel asymmetric in 5000 iterations 'STM _ ref test parallel asymmetric' failure to trigger #364
• Cygwin trunk part 2 x 2 failures with Error: Could not find the .cmi file for interface src/core/QCheck2.ml during the qcheck-core package installation.

Any idea about what's up with the Cygwin trunk part 2 failures @shym?
The strange thing is that (a) qcheck-core installed without problems on part 1 and (b) the PR doesn't touch the Cygwin trunk workflows... 🤔

Overall 7 out of 59 workflows failed, with 3 being OCaml5 issues, 2 identical test suite issues, and 2 identical infrastructure issues.

[jmid]

The Cygwin part 2 failure seems to be a general trunk regression on installing QCheck that just happened to trigger since a PR was merged to trunk on Friday in the window between the start of part 1 and the start of part 2.

The issue is now affecting all our trunk workflows. I've reported it upstream in ocaml/ocaml#12543