CI: fixup nixos-unstable-knot-* jobs

I'm not in the mood to really try understanding what's the matter,
but this change seems to work OK.

.gitlab-ci.yml

@@ -531,9 +531,7 @@ nixos-unstable-knot-dns:
   <<: *knot-dns
   <<: *nixos
   script:
-    # Knot Resolver's make-archive needs basically all build-time dependencies (meson dist),
-    # so we add them on top of the normal apkg's development shell - and run the tests.
-    - nix-shell "$(apkg srcpkg | head -n 1)" --argstr extraDepsFor knot-dns --argstr extraDeps libmnl --run 'py.test -v ci/tests/test_knot_dns.py --junitxml=report.xml'
+    - nix-shell "$(nix-shell -p apkg --run 'apkg srcpkg' | head -n 1)" --argstr extraDepsFor knot-dns --argstr extraDeps libmnl --run 'py.test -v ci/tests/test_knot_dns.py --junitxml=report.xml'
 
 
 # integration tests: Knot Resolver
@@ -611,7 +609,7 @@ nixos-unstable-knot-resolver:
   script:
     # Knot Resolver's make-archive needs basically all build-time dependencies (meson dist),
     # so we add them on top of the normal apkg's development shell - and run the tests.
-    - nix-shell "$(apkg srcpkg | head -n 1)" --argstr extraDepsFor knot-resolver --run 'py.test -v ci/tests/test_knot_resolver.py --junitxml=report.xml'
+    - nix-shell "$(nix-shell -p apkg --run 'apkg srcpkg' | head -n 1)" --argstr extraDepsFor knot-resolver --run 'py.test -v ci/tests/test_knot_resolver.py --junitxml=report.xml'
 
 
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