AptTool: add repo key before running apt-add-repository

In Ubuntu 20.04 (and maybe earlier), the behavior of

    apt-add-repository 'deb https://example.com/repo example main'

is to add the named repository to apt's sources *and* to update the
package index.  If the new repository's package key is not already in
apt's keyring, this will fail.

It is possible to change this behavior by running apt-add-repository -n,
but adding the key before running apt-add-repository also works, doesn't
require new command-line switches, and should be backwards-compatible
with apt-add-repository versions that might not behave as described
above.

conans/client/tools/system_pm.py

@@ -272,11 +272,11 @@ def installed(self, package_name):
 
 class AptTool(BaseTool):
     def add_repository(self, repository, repo_key=None):
-        _run(self._runner, "%sapt-add-repository %s" % (self._sudo_str, repository),
-             output=self._output)
         if repo_key:
             _run(self._runner, "wget -qO - %s | %sapt-key add -" % (repo_key, self._sudo_str),
                  output=self._output)
+        _run(self._runner, "%sapt-add-repository %s" % (self._sudo_str, repository),
+             output=self._output)
 
     def update(self):
         _run(self._runner, "%sapt-get update" % self._sudo_str, output=self._output)