treewide: Prevent IFD by default

Import-from-derivation (IFD) has problematic performance, and is disabled in
Nixpkgs by policy. It is arguably good practice for libraries to avoid
it whenever possible, as it has poor ergonomics in some cases,
especially with dry builds, as it requires multiple eval+build phases.

As such, prevent its use in Home Manager by default by putting existing
tests that use IFD behind a config. In CI, run a first pass with IFD
disabled, skipping tests without the config. Then run a second pass with
IFD enabled and including tests with the config. This second pass will
also run tests without the config, but they should be cached from the
previous run, so the cost is not double (only eval time should be paid
twice). It’s necessary to change from using NMT’s `run` to `build` as
`run` itself uses IFD.

Of the tests that have the config:

- kitty/theme-to-themeFile: this is a test for deprecated config, and so
  should be removed eventually anyway
- podman: the implementation relies on IFD to create individual systemd
  units from the derivation output, and so it is not straightforward to
  remove the IFD; doing so would require rethinking how the module works
  to instead have the systemd unit files included as-is rather than as
  individually configured units in the Nix config.

.github/workflows/test.yml

@@ -26,6 +26,11 @@ jobs:
     - run: ./format --ci
     - run: nix-shell --show-trace . -A install
     - run: yes | home-manager -I home-manager=. uninstall
-    - run: nix-shell -j auto --show-trace --arg enableBig false --pure tests -A run.all
+    - name: Run tests
+      run: nix-build -j auto --show-trace --arg enableBig false --pure --option allow-import-from-derivation false tests -A build.all
+      env:
+        GC_INITIAL_HEAP_SIZE: 4294967296
+    - name: Run tests (with IFD)
+      run: nix-build -j auto --show-trace --arg enableBig false --pure --arg enableLegacyIfd true tests -A build.all
       env:
         GC_INITIAL_HEAP_SIZE: 4294967296