Disable SALSA_CI_DISABLE_CROSSBUILD_ARM64 to avoid frequent errors

Disable SALSA_CI_DISABLE_CROSSBUILD_ARM64 to avoid frequent errors

As the cross-build requirement in Debian isn't universal at the moment, disable this test by default to avoid unnecessary noise. Having a CI that is consistently green, and that the "greenness" is respected, is of higher importance than having lots of things tested.

Additionally, don't allow failures on this job anymore. After this change, anyone having this job enabled by intentionally will need to fix their package (if possible) or decide to allow failures in their own pipeline.

While it would be nice to have all packages cross-build, some ecosystems are inherently not cross-buildable, and many packages are not cross-buildable while updates in the toolchain are ongoing and there is version skew between platforms. See https://crossqa.debian.net/ for latest details.

Having this test enabled by default, and failing on numerous pipelines led to an influx of bug reports to the Salsa CI pipeline, and created confusion among packagers about what the problem is and if it is serious.

Also, this test really tests the cross-build toolchain more than anything else, so packages with a failing CI could not rarely fix anything in the package itself.

Packagers can still easily test this with by triggering a manual build and setting the variable in the GitLab CI pipeline to '1' in an interface that also includes a one-line description of what the variable does.

Closes: #374 (closed)