openssl: Be Explicit About perl Usage (#16721)

The openssl build process can use the wrong perl for
various reasons, including:
* Wrong value in PERL env var
* The build process first looks for `perl5`, which the
  spack system does not provide, but some other
  distributions provide it. That way, the build process can
  end up using the wrong perl.

Stop all of these problems by explicitly setting PERL to
the to be used dependency.

var/spack/repos/builtin/packages/openssl/package.py

@@ -154,3 +154,6 @@ def link_system_certs(self):
             if os.path.isdir(sys_certs) and not os.path.islink(pkg_certs):
                 os.rmdir(pkg_certs)
                 os.symlink(sys_certs, pkg_certs)
+
+    def setup_build_environment(self, env):
+        env.set('PERL', self.spec['perl'].prefix.bin.perl)