Update README to use other actions we provide

[stertooy]

• README now refers to the install-pkg and build-pkg actions to replace the GAP_PKGS_TO_CLONE and GAP_PKGS_TO_BUILD inputs.
• The gap-version inputs in the example are now more representative of what we do in actual workflows.

[fingolfin]

On the systems where CI fails, GAP's configure prints

  checking for gcc option to enable C23 features... -std=gnu23

and then later GMP's configure runs into an error because of that.

  checking whether to enable maintainer-specific portions of Makefiles... no
  checking ABI=64
  checking compiler gcc -std=gnu23  -O2 -pedantic ... no, long long reliability test 1
  configure: error: could not find a working compiler, see config.log for details
  make: *** [/Users/runner/gap/extern/install/gmp/lib/libgmp.la] Error 1

On the good systems:

  checking for gcc option to enable C11 features... none needed

So I am guessing they changed the AC_PROG_CC macro (though on both systems I looked at the same autoconf 2.73 was used), and/or a different C/C++ compiler is installed?

[stertooy]

So I am guessing they changed the AC_PROG_CC macro (though on both systems I looked at the same autoconf 2.73 was used), and/or a different C/C++ compiler is installed?

Maybe it's because we don't run autogen.sh on releases, but do on the development version? And doing so adds the -std=gnu23 flag? And the GMP included in GAP cannot be built against that? I'm no expert on build systems so I'm just guessing here.

It worked fine a week ago:
https://github.com/gap-actions/setup-gap/actions/runs/23443260234/job/68199860277#step:4:429
But no longer does now:
https://github.com/gap-actions/setup-gap/actions/runs/23848163648/job/69520698393?pr=77#step:4:426