pkgs/build-support: refactor drvs using __structuredAttrs = true

Derivations affected by this patch set `__structuredAttrs = true;` and
provide their own `builder`, i.e. it's necessary to `source .attrs.sh`.

Rather than adding even more `if`-`source` monstrums, I decided to
modify all of those derivations to use `buildCommand` or `runCommand`,
without `builder` being set.

Then, `$stdenv/setup` is sourced already and as a result it's safe to
assume that `NIX_ATTRS_JSON_FILE`/`NIX_ATTRS_SH_FILE` point to a usable
location both in a build and a shell session.

pkgs/build-support/binary-cache/make-binary-cache.py

@@ -3,7 +3,7 @@ import json
 import os
 import subprocess
 
-with open(".attrs.json", "r") as f:
+with open(os.environ["NIX_ATTRS_JSON_FILE"], "r") as f:
   closures = json.load(f)["closure"]
 
 os.chdir(os.environ["out"])