Motivation

build.nix fails when it selects packages that have disabled Python pkgs (like with disabled = !isPy3k;) as dependencies.

Example

nix-build --show-trace  maintainers/scripts/build.nix --argstr maintainer earvstedt
# Output
error: ... while evaluating ... 'python2.7-djangoql-0.14.0'
...
Django-2.2.15 not supported for interpreter python2.7

Explanation:
python27Packages.djangoql is not disabled but has the disabled Py3k-only dependency django.

Discussion

To check which pkgs should be built, build.nix only evaluates the meta.maintainers attribute of each pkg.
If the maintainer doesn't match or if evaluation of meta.maintainers fails, the package is skipped.
When a pkg is selected to be built but one of its dependencies is disabled the build fails.

The workaround to skip all packages whose closure doesn't evaluate is not ideal as this may ignore actual package defects.
But most of these potential eval failures should already get caught by ofborg before merging.

This is relevant for ZHF: #97479

I'll add a backport in case this PR gets merged.