build_library: fix Exoscale image format, size, and compression

[jheidel]

Fix Exoscale image format, size, and compression

Exoscale requires Custom Templates to be at least 10GB, expects a .qcow2 extension, and Exoscale Custom Templates do not support compressed (.bz2) images. Currently, Flatcar releases an 8GB compressed .img.bz file.

This change ensures the output file uses the .qcow2 extension, applies a post-conversion resize to exactly 10G, and disables compression in the release pipeline, matching that of similar platforms (scaleway, etc).

This fixes flatcar/Flatcar#2163

How to use

Reviewers can verify that the generated .qcow2 image can be directly used by Exoscale Custom Template and that this instance can be used to create a new compute instance with 10G disk size.

Testing done

The changes were verified using the Flatcar SDK:

1. Build base image and oem sysexts:

./run_sdk_container ./build_image --board=amd64-usr --getbinpkg
./run_sdk_container ./build_image --board=amd64-usr --getbinpkg oem_sysext

2. Generate Exoscale image:

./run_sdk_container ./image_to_vm.sh --board=amd64-usr --getbinpkg --format=exoscale --image_compression_formats=none

3. Verify image metadata:

   qemu-img info __build__/images/amd64-usr/latest/flatcar_production_exoscale_image.qcow2

   image: flatcar_production_exoscale_image.qcow2
   file format: qcow2
   virtual size: 10 GiB (10737418240 bytes)
   

4. Real-world test: The resulting image was hosted on a public web server and successfully imported into Exoscale's 'Add Custom Template' UI without any manual renaming, resizing, or decompression required.