pmbootstrap repo_missing: doesn't list networkmanager-qt for x86_64

$ pmbootstrap -v repo_missing --built networkmanager-qt
[19:46:53] Calculate packages that need to be built (networkmanager-qt, x86_64) - this may take some time
[19:46:58] ERROR: networkmanager-qt can't be built for x86_64. Either itself or one if its dependencies is not available for that architecture. Run with -v for details.
[19:46:58] See also: <https://postmarketos.org/troubleshooting>

Relevant log lines:

(004364) [19:46:58] mesa: (indirectly) depends on networkmanager-qt
(004364) [19:46:58] mesa: can't be built for x86_64
(004364) [19:46:58] ERROR: networkmanager-qt can't be built for x86_64. Either itself or one if its dependencies is not available for that architecture. Run with -v for details.

pmbootstrap should know that it can use Alpine's mesa package, which does exist for x86_64, although our mesa pmaport purposefully does not exist for x86_64.

(It is probably a good idea to add a CI check for all packages: are their depends satisfied for all architectures?)