Does my NixOS 21.11 build run forever?

I’ve been building my NixOS 21.05 configuration successfully for a while now, but after changing to 21.11 the build seems to run much longer, going from a few minutes to over three hours, at which point the CI pipeline times out. It looks like an infinite loop. What’s the likely culprit?

The last lines of the output:

building '/nix/store/hkbrdj0ik1mh24pva62bc2qgh102s7sr-nixos-boot-disk.drv'...
Formatting '/nix/store/a40mcsv5v7ni3a4r432bz41jhvnal4az-nixos-boot-disk/disk.img', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=62914560 lazy_refcounts=off refcount_bits=16
EFI stub: Loaded initrd from LINUX_EFI_INITRD_MEDIA_GUID device path

The build succeeds locally, when not running in the Docker container and with some actual common.nix, hardware-configuration.nix, and host.nix contents. When running the exact CI steps locally I can reproduce the blocking behaviour.

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