Fix CI merge error by disabling windows workflow on pushes to master

[jmid]

Since the merge of #29040 including 703ece7 I've been getting an error email every time I merge a PR.

(The email has Subject: [ocaml/opam-repository] Run failed: Windows CI - master (9684018)
and a body that says "Windows CI: All jobs were successful" ... but I digress 😅 )

Looking at the GitHub Actions CI interface, I observed that Get changed files is not run on a merge commit, since it is guarded by if: github.event_name != 'push', but the subsequent steps depending on the changed files info are still enabled:

Up until 703ece7 the subsequent steps were similarly guarded, so I started by inserting those again.
I then realized that running a workflow on pushes to master only to hit a series of disabled workflow steps that perform any actual checks makes little sense.

This PR therefore disables the windows CI workflow on pushes to master (incl.merge) where checks are disabled already.
It should only make a difference to opam maintainers who hopefully won't get an error email after this.

[jmid]

OK, this needed a bit back-and-forth but now it should be ready:

• Since 703ece7 splits the Windows CI into an analyse and (dynamic) build jobs, the latter needed a guard to handle the "no packages" case. Previously we had only 2 jobs with inter-job-step-dependencies already handling this.
• On merges to master, the Get changed files step was disabled, but the remaining steps weren't, which caused the error.
• When choosing between "Run a workflow with disabled checks" and "Don't run the workflow" in this case I maintain that the latter is the reasonable choice.

In summary:

• the PR disables a useless job on merges to master
• it adds a guard to handle the "no changed/added packages" case
• this fixes the annoying error emails on merge-to-master
• this should also handle PRs such as As proposed in #29060, set the default to "(latest)". #29074 with no packages/ changes, which appear green, but which I suspect is also triggering an annoying error email from the looks of its GHA summary page: https://github.com/ocaml/opam-repository/actions/runs/20136309689?pr=29074

[mseri]

Thanks