Define the matrix for the others job using an actions/github-script
step, which allows the possibility of the matrix being dynamic.

Tests a full `--disable-shared` build on Linux and also a Linux build
with as many options disabled as possible (as the minimal build in the
other-configs job on Jenkins also does).

The matrix is expanded by adding the 'CI: Full matrix' label to a pull
request.

If Cygwin is running "elevated" - which it is in CI - then it acts as
though it's running as root. It intentionally activates
SeBackupPrivilege, which thwarts the test_create_cursor_failures.ml
test.

The OCaml testsuite will never require root privileges for anything
meaningful, so ocamltest on Cygwin simply drops the SeBackupPrivilege
when running on Cygwin, which means the test correctly fails.

Applying this label _before_ the testsuite has started in either GitHub
Actions or AppVeyor will cause the execution of the testsuite to be
skipped.