treewide: remove uses of now-removed nixUnstable

nixUnstable was removed in 2b4e18f3d4
but is still referred to from a variety of places, including Hydra's
cross-trunk jobset:

```
in job ‘armv7l-hf.nixUnstable.aarch64-linux’:
error:
       … while evaluating a branch condition
         at /nix/store/m549zq6lcl10d1jqcdgcdaxxj9xdhxfv-source/lib/customisation.nix:479:5:
          478|     in
          479|     if drv == null then null else deepSeq drv' drv';
             |     ^
          480|

       … in the left operand of the update (//) operator
         at /nix/store/m549zq6lcl10d1jqcdgcdaxxj9xdhxfv-source/lib/meta.nix:43:9:
           42|   addMetaAttrs = newAttrs: drv:
           43|     drv // { meta = (drv.meta or {}) // newAttrs; };
             |         ^
           44|

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: nixUnstable has been removed. For bleeding edge (Nix
       master, roughly weekly updated) use nixVersions.git, otherwise
       use nixVersions.latest.
```

doc/using/configuration.chapter.md

@@ -193,7 +193,7 @@ source: ../config-options.json
 
 ### Build an environment {#sec-building-environment}
 
-Using `packageOverrides`, it is possible to manage packages declaratively. This means that we can list all of our desired packages within a declarative Nix expression. For example, to have `aspell`, `bc`, `ffmpeg`, `coreutils`, `gdb`, `nixUnstable`, `emscripten`, `jq`, `nox`, and `silver-searcher`, we could use the following in `~/.config/nixpkgs/config.nix`:
+Using `packageOverrides`, it is possible to manage packages declaratively. This means that we can list all of our desired packages within a declarative Nix expression. For example, to have `aspell`, `bc`, `ffmpeg`, `coreutils`, `gdb`, `nix`, `emscripten`, `jq`, `nox`, and `silver-searcher`, we could use the following in `~/.config/nixpkgs/config.nix`:
 
 ```nix
 {
@@ -206,7 +206,7 @@ Using `packageOverrides`, it is possible to manage packages declaratively. This
         coreutils
         gdb
         ffmpeg
-        nixUnstable
+        nix
         emscripten
         jq
         nox
@@ -230,7 +230,7 @@ To install it into our environment, you can just run `nix-env -iA nixpkgs.myPack
         coreutils
         gdb
         ffmpeg
-        nixUnstable
+        nix
         emscripten
         jq
         nox
@@ -258,7 +258,7 @@ After building that new environment, look through `~/.nix-profile` to make sure
         bc
         coreutils
         ffmpeg
-        nixUnstable
+        nix
         emscripten
         jq
         nox
@@ -292,7 +292,7 @@ This provides us with some useful documentation for using our packages.  However
         coreutils
         ffmpeg
         man
-        nixUnstable
+        nix
         emscripten
         jq
         nox
@@ -344,7 +344,7 @@ Configuring GNU info is a little bit trickier than man pages. To work correctly,
         coreutils
         ffmpeg
         man
-        nixUnstable
+        nix
         emscripten
         jq
         nox

pkgs/build-support/build-fhsenv-bubblewrap/default.nix

@@ -93,7 +93,7 @@ let
       files = [
         # NixOS Compatibility
         "static"
-        "nix" # mainly for nixUnstable users, but also for access to nix/netrc
+        "nix" # mainly for nixVersions.git users, but also for access to nix/netrc
         # Shells
         "shells"
         "bashrc"

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.8

@@ -272,7 +272,7 @@ relevant configuration options.
 Normally,
 .Nm
 first builds the
-.Ql nixUnstable
+.Ql nix
 attribute in Nixpkgs, and uses the resulting instance of the Nix package manager
 to build the new system configuration. This is necessary if the NixOS modules
 use features not provided by the currently installed version of Nix. This option

pkgs/top-level/release-cross.nix

@@ -5,7 +5,7 @@
 
    e.g.
 
-   $ nix-build pkgs/top-level/release-cross.nix -A crossMingw32.nixUnstable --arg supportedSystems '[builtins.currentSystem]'
+   $ nix-build pkgs/top-level/release-cross.nix -A crossMingw32.nix --arg supportedSystems '[builtins.currentSystem]'
 
    To build all of the bootstrapFiles bundles on every enabled platform, use:
 
@@ -70,7 +70,7 @@ let
     gmp = nativePlatforms;
     libcCross = nativePlatforms;
     nix = nativePlatforms;
-    nixUnstable = nativePlatforms;
+    nixVersions.git = nativePlatforms;
     mesa = nativePlatforms;
     rustc = nativePlatforms;
     cargo = nativePlatforms;