Install Mail::SPF manually in Github Actions

There is a long-standing packaging issue with Mail::SPF that prevents
cpanm from automatically installing it with non-root privileges. [1] It
also seems that upstream is unwilling to patch the problem.

Yet this package is very useful for checking SPF syntax. One does not
simply reimplement the same thing by oneself.

So what are our options? Fork it? Copy the code and maintain it in a
Zonemaster::Engine::SPF namespace or similar? Or work around the bug?

Fortunately, the Perl library in question is easy to find in
distribution package repositories. It’s packaged by Debian, Ubuntu,
Alpine Linux, FreeBSD and Red Hat based OSes at least.

However, in Github Actions, all Perl modules are installed from CPAN.
After discussion, this is too far away from the documented installation
instructions and should be changed in some other pull request, in order
to favor OS-supplied packages instead when possible. That’s why this
commit installs Mail::SPF from packages.

[1]: https://rt.cpan.org/Public/Bug/Display.html?id=34768

.github/workflows/ci.yml

@@ -49,6 +49,7 @@ jobs:
             gettext \
             libidn2-dev \
             liblog-any-perl \
+            libmail-spf-perl \
             libssl-dev \
             libtool \
             m4 \