AptTool: add repo key before running apt-add-repository (#8857)

[hannahwhy]

Changelog: Bugfix: AptTool: add repo key before running apt-add-repository.
Docs: omit

Closes #8857

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.

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[CLAassistant]

All committers have signed the CLA.

[hannahwhy]

As described in #8857, this proposed change will have AptTool running an extra index update, which does waste some time. However:

1. The approach here doesn't change the interface expected from apt-add-repository.
2. Two index updates already occur under Ubuntu 20.04 without this change.

The version of apt-add-repository in Ubuntu 20.04 has a -n, --no-update option that skips the index update. If we use that, then the order of operations in AptTool can remain the way they are. However, I have yet not confirmed that -n and/or --no-update exist in all versions of apt-add-repository, not only in Debian and Ubuntu but also other Debian derivatives.

[hannahwhy]

Looks like I'm running afoul of some expected command sequences in the test suite. I'll check those in a little while.

[memsharded]

Note for the reviewers:

• The disadvantage of this solution is that it will run an "update" twice, second might be redundant, and it might be less efficient. This doesn't happen for all cases (only when adding repository and providing key), but better to be sure impact is reasonable.

[lasote]

I think the added time is reasonably low and probably we don't have a "safer" way to do it without breaking. For me, it could be merged.