cirrus: Look up existing versions of python dependencies.

FreeBSD sometimes changes the base version of python3 that is
used for packages.  This affects package names.  For example,
currently CI is broken, because there is no more py37- versions
of sphinx and openssl available, only py38- ones:

  pkg: No packages available to install matching 'py37-openssl'
       have been found in the repositories
  pkg: No packages available to install matching 'py37-sphinx'
       have been found in the repositories

We had the same issue last year with 3.6 -> 3.7 transition:
  dfa2e3d ("cirrus: Use python 3.7 packages on FreeBSD.")

Fixing that by searching for a package instead of using a specific
version.  This should help to avoid same issues in the future.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>

.cirrus.yml

@@ -8,8 +8,8 @@ freebsd_build_task:
     memory: 8G
 
   env:
-    DEPENDENCIES: automake libtool gmake gcc wget openssl
-                  python3 py37-openssl py37-sphinx
+    DEPENDENCIES: automake libtool gmake gcc wget openssl python3
+    PY_DEPS:      sphinx|openssl
     matrix:
       COMPILER: gcc
       COMPILER: clang
@@ -18,6 +18,7 @@ freebsd_build_task:
     - sysctl -w kern.coredump=0
     - pkg update -f
     - pkg install -y ${DEPENDENCIES}
+        $(pkg search -xq "^py3[0-9]+-(${PY_DEPS})-[0-9]+" | xargs)
 
   configure_script:
     - ./boot.sh