Move GitHub action workflows to 5.1.1

[jmid]

This PR moves the GitHub action workflows from 5.1.1~rc1 to 5.1.1.

Since this release includes two assertion error fixes, this should take care of (some of) the failures we have been seeing.

[jmid]

CI summary:

• Cygwin 5.1 timed out during Lin Dynlink #307
• Cygwin trunk aborted during domain_spawntree #354
• Linux trunk debug triggered multiple domain.c assertion failures #424
• Linux trunk debug triggered multiple domain.c assertion failures #424
• MinGW bytecode 5.1 timed out during threadomain #203
• MinGW bytecode trunk failed with Fatal error: Failed to create domain during domain_spawntree - with Atomic #354
• MinGW trunk failed - but is missing a CI log!
• macos-arm64-5.0 failed with Internal error (due to a missing 5.0 image)
• freebsd-amd64-5.0 failed with Internal error (due to a missing 5.0 image)
• freebsd-amd64-5.1 failed to find a counterexample in Lin Out_channel test with Domain #401

Out of 59 workflows 10 failed with 6 genuine, 3 CI defects, and 1 false alarm

[jmid]

CI summary for merge to main:

• Cygwin 5.1 failed due to a faulty libgpg-error0 released on Cygwin affecting curl and opam
• Cygwin trunk failed due to a faulty libgpg-error0 released on Cygwin affecting curl and opam
• MinGW bytecode trunk failed due to a faulty libgpg-error0 released on Cygwin affecting curl and opam
• MinGW trunk failed due to a faulty libgpg-error0 released on Cygwin affecting curl and opam
• linux-arm64-5.2 timed out inthread_createtree - with Atomic #425
• linux-s390x-5.2 failed to find a counterexample for STM Weak test parallel #406

Out of 36 workflows 6 failed with 1 genuine error, 1 false positive, and 4 CI errors