test-builder.sh: Allow Appstream icons to be in PNG or JPEG-XL format

[smcv]

Older versions of the Appstream library always used PNG here, but 1.2.0 uses JPEG-XL.

Resolves: #776

[bbhtt]

It's going to be problematic if the default format changed to JXL with 1.2.0. It sounds like a backwards incompatible change from Appstream to me. We probably need to keep PNG at least for stable.

I was planning to add an arg to control jxl/png output later but I didn't realise the default had changed.

[bbhtt]

Mind testing #779? I don't have CI or local with AppStream 1.2.0

[smcv]

It's going to be problematic if the default format changed to JXL with 1.2.0. It sounds like a backwards incompatible change from Appstream to me.

Clearly the authors of Appstream don't think it's problematic, and we've already had a stable release of flatpak-builder (a security fix release, even) with the new behaviour. I'd really prefer to make a minimal change to get the tests passing again before making behaviour changes, even if we subsequently change it back to PNG as well (at which point the tests can become stricter again).

If you disagree, I can try to make #779 work, but the amount of time I can spend on flatpak-builder is rather limited (it's very much a secondary thing for me compared with flatpak and bubblewrap).

[bbhtt]

I won't stop you from using this patch downstream temporarily but for upstream purposes I'll go with #779.

I'm aware of the problems re. the security release, it's why I rushed to create #779 in weekdays.

In any case I'll cut a new release with #779 by the end of this week, once the CI situation is figured out.

[bbhtt]

Closing in favour of #779