Add support for snapcraft 7 credentials

[Tony Espy]

Snapcraft 7 introduced a new format for credentials generated by the export-login command, however the latest version of ubuntu-image (2.2+snap9) isn't able to parse the new format. This means someone who wants to build an Ubuntu Core 22 image that requires access to snaps in a brand store must install snapcraft 6 to export login credentials.

For reference: https://forum.snapcraft.io/t/snapcraft-authentication-options/30473

Steps to reproduce:
1. Install snapcraft 7.1.1 from latest/stable
2. Export login credentials: snapcraft export-login --acls package_access mycreds
3. Download a core22 reference model assertion from: https://github.com/snapcore/models
4. Install ubuntu-image from latest/stable
5. Build the image:

UBUNTU_STORE_AUTH_DATA_FILENAME=./mycreds ubuntu-image snap <your-model-assertion>
Error: Error preparing image: cannot decode auth file "./mycreds"

That said, I should've looked closer at the error, it looks like this is probably a snapd bug (i.e. in 'snap prepare-image') vs. a bug in ubuntu-image. If you agree, I'll move the bug.

[William Wilson (jawn-smith)]

Thanks for the steps to reproduce! This appears to be a bug in snapd's image.Prepare function. It seems to need updating to use the snapcraft v7 credentials. I've tried recompiling ubuntu-image with snapd 2.57 but that did not resolve the issue. As soon as this is fixed in snapd I can update go.mod to pull in the change.

[Samuele Pedroni (pedronis)]

https://github.com/snapcore/snapd/pull/12065 has the new support

[Samuele Pedroni (pedronis)]

the fix will be in 2.57.2

snapd will now also support UBUNTU_STORE_AUTH that accepts the same values (the creds/auth string, not a file) like SNAPCRAFT_STORE_CREDENTIALS

[William Wilson (jawn-smith)]

The ubuntu-image in latest/edge (rev 317) should have this fix.

[Tony Espy (awe)]

I tested ubuntu-image (r317 from edge) along with snapd 2.57.2 (r17029) from latest/beta and can confirm that credentials exported from snapcraft 7 now work!