snapcraft-managed lxd containers do not respect parallel installs

[Alex Lewontin]

In the case when multiple instances of snapcraft are installed in parallel (https://snapcraft.io/docs/parallel-installs) instances suffixed with an instance key (e.g. _foo) appear to install the unsuffixed instance from the host into their managed lxd container. This then results in build failure.

Steps to reproduce:

1) enable parallel installs as described here: https://snapcraft.io/docs/parallel-installs
2) run `snap install snapcraft --channel=6.x/stable --classic` (revision 7201 at reporting time)
3) run `snap install snapcraft_7 --channel=7.x/stable --classic` (revision 8093 at reporting time)
4) initialize a fresh snap using `snapcraft init` and change the base to core22
5) try to run `snapcraft_7` to build that snap

$ snapcraft_7
Launching instance...
Usage: snapcraft pack [OPTIONS] DIRECTORY
Try 'snapcraft pack -h' for help.

Error: No such option: --build-for
Failed to execute pack in instance.
Full execution log: '/home/alex/.cache/snapcraft/log/snapcraft-20220811-135653.968445.log'

Execution log attached to bug report
Lines of interest: 63, 66

I have not tested this using other permutations of versions across parallel installs, or using multipass