Disable the 'cygwin' GitHub workflow

It does not work at all. For example, `rpm -E %fedora` says that there
should be version 33 of rpmsphere at
https://github.com/rpmsphere/noarch/tree/master/r, but there is only
version 32.

Another thing that is broken: Cygwin now assumes that a recent
mingw-w64-headers version is available, but Fedora apparently only
offers v7.0.0, which is definitely too old to accommodate for the
expectation of cygwin/cygwin@c1f7c4d1b6d7.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/workflows/cygwin.yml

@@ -1,6 +1,6 @@
 name: cygwin
 
-on: push
+on: workflow_dispatch
 
 jobs:
   fedora-build: