For example, this prevents a "kvm" build on machines that don't have
KVM.

Fixes #2012.

This is primarily because Derivation::{can,will}BuildLocally() depends
on attributes like preferLocalBuild and requiredSystemFeatures, but it
can't handle them properly because it doesn't have access to the
structured attributes.

E.g. __noChroot and allowedReferences now work correctly. We also now
check that the attribute type is correct. For instance, instead of

  allowedReferences = "out";

you have to write

  allowedReferences = [ "out" ];

Fixes #2453.

This meant that making a typo in an s3:// URI would cause a bucket to
be created. Also it didn't handle eventual consistency very well. Now
it's up to the user to create the bucket.

Tools which re-exec `$SHELL` or `$0` or `basename $SHELL` or even just
`bash` will otherwise  get the non-interactive bash, providing a
broken shell for the same reasons described in
https://github.com/NixOS/nixpkgs/issues/27493.

Extends c94f3d55

Presumably this refers to ./default.nix but the support for that in
'nix' is tenuous. Also folders are a Mac thing.

* Don't wait forever for the client to remove data from the
  buffer. This does mean that the buffer can grow without bounds
  (e.g. when downloading is faster than writing to disk), but meh.

* Don't hold the state lock while calling the sink. The sink could
  take any amount of time to process the data (in particular when it's
  actually a coroutine), so we don't want to block the download
  thread.

In particular this causes copyStorePath() from HttpBinaryCacheStore to
only start a download if needed. E.g. if the destination LocalStore
goes to sleep waiting for the path lock and another process creates
the path, then LocalStore::addToStore() will never read from the
source so we don't have to do the download.

Changes

  std::bad_alloc

into

  bad archive: input doesn't look like a Nix archive

(cherry picked from commit a94a2eb1cb1c81e90a7529be5fecac27899a3442)

Fixes #2425.

Otherwise, we just keep asking the substituter for other .narinfo
files, which can take a very long time due to retries/timeouts.

Fixes #1990.

We shouldn't pollute stdout.

Fixes #2393.

‘geteuid’ gives us the user that the command is being run as,
including in setuid modes. By using geteuid to determind id, we can
avoid the ‘sudo -i’ hack when upgrading Nix. So now, upgrading Nix on
macOS is as simple as:

$ sudo nix-channel --update
$ sudo nix-env -u

$ sudo launchctl stop org.nixos.nix-daemon
$ sudo launchctl start org.nixos.nix-daemon
or
$ sudo systemctl restart nix-daemon

This is already done by coerceToString(), provided that the argument
is a path (e.g. 'fetchGit ./bla'). It fixes the handling of URLs like
git@github.com:owner/repo.git. It breaks 'fetchGit "./bla"', but that
was never intended to work anyway and is inconsistent with other
builtin functions (e.g. 'readFile "./bla"' fails).

Fixes #2390.

https://hydra.nixos.org/build/80480356

Also some cosmetic improvements.

E.g.

  $ nix upgrade-nix
  error: directory '/home/eelco/Dev/nix/inst/bin' does not appear to be part of a Nix profile

instead of

  $ nix upgrade-nix
  error: '/home/eelco/Dev/nix/inst' is not a symlink

Fix a 32-bit overflow that resulted in negative numbers being printed;
use fmt() instead of boost::format(); change -H to -h for consistency
with 'ls' and 'du'; make the columns narrower (since they can't be
bigger than 1024.0).

If the user has an object greater than 1024 yottabytes, it'll just display it as
N yottabytes instead of overflowing.

Swaps to use boost::format strings instead of std::setw and std::setprecision.

Unfortunately, -h is already taken as a short option by --help, so we have to
use a different letter or the capitalized version.

Resolves #2363