Support flake references in the old CLI

Fixes #7026.

doc/manual/src/command-ref/env-common.md

@@ -8,41 +8,10 @@ Most Nix commands interpret the following environment variables:
 
   - [`NIX_PATH`]{#env-NIX_PATH}\
     A colon-separated list of directories used to look up Nix
-    expressions enclosed in angle brackets (i.e., `<path>`). For
-    instance, the value
-
-        /home/eelco/Dev:/etc/nixos
-
-    will cause Nix to look for paths relative to `/home/eelco/Dev` and
-    `/etc/nixos`, in this order. It is also possible to match paths
-    against a prefix. For example, the value
-
-        nixpkgs=/home/eelco/Dev/nixpkgs-branch:/etc/nixos
-
-    will cause Nix to search for `<nixpkgs/path>` in
-    `/home/eelco/Dev/nixpkgs-branch/path` and `/etc/nixos/nixpkgs/path`.
-
-    If a path in the Nix search path starts with `http://` or
-    `https://`, it is interpreted as the URL of a tarball that will be
-    downloaded and unpacked to a temporary location. The tarball must
-    consist of a single top-level directory. For example, setting
-    `NIX_PATH` to
-
-        nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz
-
-    tells Nix to download and use the current contents of the
-    `master` branch in the `nixpkgs` repository.
-
-    The URLs of the tarballs from the official nixos.org channels (see
-    [the manual for `nix-channel`](nix-channel.md)) can be abbreviated
-    as `channel:<channel-name>`.  For instance, the following two
-    values of `NIX_PATH` are equivalent:
-
-        nixpkgs=channel:nixos-21.05
-        nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz
-
-    The Nix search path can also be extended using the `-I` option to
-    many Nix commands, which takes precedence over `NIX_PATH`.
+    expressions enclosed in angle brackets (i.e., `<path>`),
+     e.g. `/home/eelco/Dev:/etc/nixos`. It can be extended using the
+    `-I` option. For more information about the semantics of the Nix
+    search path, see the documentation for `-I`.
 
   - [`NIX_IGNORE_SYMLINK_STORE`]{#env-NIX_IGNORE_SYMLINK_STORE}\
     Normally, the Nix store directory (typically `/nix/store`) is not

doc/manual/src/release-notes/rl-next.md

@@ -1,2 +1,10 @@
 # Release X.Y (202?-??-??)
 
+* You can now use flake references in the old CLI, e.g.
+
+  ```
+  # nix-build flake:nixpkgs -A hello
+  # nix-build -I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05 \
+      '<nixpkgs>' -A hello
+  # NIX_PATH=nixpkgs=flake:nixpkgs nix-build '<nixpkgs>' -A hello
+  ```