NixOS/nixpkgs

This shaves 47 MiB off the closure size. (libboost_context.so is only
19 KiB.)

This causes collisions between the build outputs of `nix` when building
in an environment with `nix1.perl-bindings` and `nix`:

```
collision between `/nix/store/aa4rrcj7dg2xj4rfkiclcmp745ibqng0-nix-2.0.4/lib/libnixstore.so' and `/nix/store/sp0sdi4bll80h58big1iy8kkh3qqxpw2-nix-1.11.16/lib/libnixstore.so'
builder for '/nix/store/wgbccin107lhm8cv9imnnvkx1j2pgibc-hydra-perl-deps.drv' failed with exit code 25
```

Unlike on linux these are not namespaced per user so this will cause
build failures if /tmp/nix-test was not removed by a previous build if
the nixbld user id doesn't match by accident. Nix already creates a
unique tempdir for builds so we can use that instead.

Fixes #44172

This makes the command ‘nix-env -qa -f. --arg config '{skipAliases =
true;}'’ work in Nixpkgs.

Misc...

- qtikz: use libsForQt5.callPackage

  This ensures we get the right poppler.

- rewrites:

  docbook5_xsl -> docbook_xsl_ns
  docbook_xml_xslt -> docbook_xsl

diffpdf: fixup

(stdenv.system is the system running the build, apparently)

The isSeccomputable flag treated Linux without seccomp as just a
normal variant, when it really should be treated as a special case
incurring complexity debt to support.

cc NixOS/nix#1951

Important security patch for no-seccomp systems.

It's the last staging commit (mostly) built on Hydra,
and a minimal fix for Darwin regression in pysqlite.

Older nix versions likely aren't feasible due to the perl dependency.