mk-aggregated: use buildEnv

[szlend]

The symlinkJoin builder might be lighter on dependencies, but it actually creates a very inefficient directory structure, symlinking literally every file in the toolchain. This results in ~47k symlinks created with the default toolchain and takes a while on slower file systems.

The buildEnv builder depends on perl and is a bit awkward to use with propagated inputs, but it creates a much more efficient directory structure.

With symlinkJoin:

find ./result/ | wc -l
47006

With buildEnv:

find ./result/ | wc -l
80

[oxalica]

The symlinkJoin builder might be lighter on dependencies, but it actually creates a very inefficient directory structure, symlinking literally every file in the toolchain. This results in ~47k symlinks created with the default toolchain and takes a while on slower file systems.

That's definitely a problem I want to fix for a long time. But I don't know the "proper" method to produce the minimal symlink tree. It's good to know that buildEnv does a better job here.

The buildEnv builder depends on perl and is a bit awkward to use with propagated inputs

I agree. Since the aggregated drv is almost always built on users' machine, this change will force all users to download perl to build any toolchain, which is less ideal. But I think build-only dependency to perl is probably still better than persistent thousands of symlinks after-build.

Ideally, we would like to optimize upstream implementation of symlinkJoin.

[lyynd]

The approach in this PR has an additional benefit of fixing #221 (just tested).