Remove the --delete option for --gc. Fixes #3343

Running `nix-store --gc --delete` will, as of Nix 2.3.3, simply fail
because the --delete option conflicts with the --delete operation.

  $ nix-store --gc --delete
  error: only one operation may be specified
  Try 'nix-store --help' for more information.

Furthermore, it has been broken since at least Nix 0.16 (which was
released sometime in 2010), which means that any scripts which depend
on it should have been broken at least nine years ago. This commit
simply formally removes the option. There should be no actual difference
in behaviour as far as the user is concerned: it errors with the exact
same error message. The manual has been edited to remove any references
to the (now gone) --delete option.

Other information:
* Path for Nix 0.16 used:
  /nix/store/rp3sgmskn0p0pj1ia2qwd5al6f6pinz4-nix-0.16

See 0629601d.

Fixes #3175.

Everything uses the generic caching system now.

This is not compatible with lazy flake input fetching.

If you do a fetchTree on a Git repository, whether the result contains
a revCount attribute should not depend on whether that repository
happens to be a shallow clone or not. That would complicate caching a
lot and would be semantically messy. So applying fetchTree/fetchGit to
a shallow repository is now an error unless you pass the attribute
'shallow = true'. If 'shallow = true', we don't return revCount, even
if the repository is not actually shallow.

Note that Nix itself is not doing shallow clones at the moment. But it
could do so as an optimisation if the user specifies 'shallow = true'.

Issue #2988.

Also, don't return a revCount anymore for shallow or dirty Git trees,
since it's incorrect.

Closes #2988.

Add missing `#include <regex>`

display attr-path only when queried available

The OpenSSL files were removed in a6ca68a7.

https://salsa.debian.org/debian/nix/issues/3

Unless the 'tested' job in the Nixpkgs/NixOS jobsets, this job isn't
actually used for anything (e.g. we don't update a channel based on
whether 'release' succeeds).

This makes it build on non-x86_64-linux systems (needed in GitHub
actions).

Remove the tarball job

Source tarballs are not very useful anymore. People who want to build
from source can also just build from the Git repository. Once upon a
time, the source tarball also saved users from needing a few
dependencies (e.g. bison and flex) but those are dwarfed by the other
dependencies, so it's no longer worth it.

Note: the release script should be updated to copy the vendoredCrates
tarball.

Currently the build fails with

  warning: reject refs/heads/HEAD because shallow roots are not allowed to be updated
  error: Could not read 0c2088d4
  fatal: Failed to traverse parents of commit ea1803ef
  error: program 'git' failed with exit code 128

Add CI with github actions

Fixes #3396.

This file is licensed under the GPL. Originally, Nix was also
GPL-licensed so that was fine. However, we later changed the license
to the LGPL but missed the fact that style.css has an incompatible
license.

Since the Nix manual at nixos.org uses its own styling, we can remove
this file.

Fixes #3392.

The make-rules repo is not maintained.

Fixes #3406.