Install Mail::SPF manually in Travis

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 at least.

However, in Travis, all Perl modules are install from CPAN, so in order
to keep that consistency, the best option is to install Mail::SPF from
CPAN while adding the workaround documented in the library’s INSTALL
file. Does it work? Let’s find out.

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

.github/workflows/ci.yml

@@ -53,6 +53,12 @@ jobs:
             libtool \
             m4 \
 
+        # Work around bug in Mail::SPF preventing normal installation with cpanm
+        # (see https://rt.cpan.org/Public/Bug/Display.html?id=34768)
+      - name: Install Mail::SPF (latest)
+        run: |
+          cpanm --notest --install-args="--install_path sbin=$HOME/.local/sbin" Mail::SPF
+
       - name: Install Zonemaster::LDNS (latest)
         if: ${{ matrix.compatibility == 'latest' }}
         run: |