I noticed that my system closure contained duplicate of various
packages, and traced it back to `googleearth-pro`. It included a
`env-vars` file created by `stdenv` which pulled in lots of build tools,
and it seems it was copied in `installPhase` by accident.
By only copying the directories from the upstream package, the number of paths
in the closure is reduced noticably:
```diff
~/build/nixpkgs $ diff -u <(nix-store --query -R result-before|wc -l) <(nix-store --query -R result|wc -l)
--- /dev/fd/63 2023-04-23 14:31:22.653577750 +0200
+++ /dev/fd/62 2023-04-23 14:31:22.654577714 +0200
@@ -1 +1 @@
-396
+256
$ du -sch $(nix-store -qR result-before) | grep total
1.5G total
$ du -sch $(nix-store -qR result) | grep total
894M total
```
