disabling the CI on Cygwin

[Bruno Haible]

The newest Cygwin release (3.5.5) has three major regressions:
  - bash hangs [1]
  - access() behaviour changed [2]
  - raise() produces random behaviour [3]

The first one has the potential to hang the CI for 6 hours; the second and
third one causes unit test failures in the CIs of gnulib, gettext, and m4.

Since the GitHub CI picks up the newest Cygwin release automatically
and I don't want to spend hours again, hunting down Cygwin regressions
in basic POSIX functionality, I am disabling the Cygwin (64-bit) CI for
these packages.

I keep the Cygwin (32-bit) CI, since that is running a fixed release (3.3.6) —
no risk of regressions caused by Cygwin.

Bruno

[1] https://sourceware.org/pipermail/cygwin/2024-December/256971.html
[2] https://sourceware.org/pipermail/cygwin/2024-December/256972.html
[3] https://sourceware.org/pipermail/cygwin/2024-December/256973.html