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 <[email protected]>

Author: dscho

.github/workflows/cygwin.yml

@@ -1,12 +1,6 @@
 name: cygwin
 
-on:
- push:
-   # since master is a symbolic reference to main, don't run for both
-   branches-ignore:
-     - 'master'
-   tags:
-     - '*'
+on: workflow_dispatch
 
 jobs:
   fedora-build: