.cirrus.yml: bootstrap pkg unconditionally

This ensures compatibility with pkg repo so a change in upstream
doesn't break setup. See:

  https://lists.freebsd.org/pipermail/freebsd-cloud/2020-April/000234.html

Message-Id: <CAKBkRUzicxphjjkkxdgzB3cDSv=AszD5V4X499jT2BjiAaazGw@mail.gmail.com>
Signed-off-by: Li-Wen Hsu <lwhsu@lwhsu.org>
[AJB: applied from Li-Wen's github, applied sob, tweaked commit message]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.cirrus.yml

@@ -6,7 +6,7 @@ freebsd_12_task:
     image_family: freebsd-12-1
     cpu: 8
     memory: 8G
-  install_script: pkg install -y
+  install_script: ASSUME_ALWAYS_YES=yes pkg bootstrap -f ; pkg install -y
     bash bison curl cyrus-sasl git glib gmake gnutls gsed
     nettle perl5 pixman pkgconf png usbredir
   script: