Merge pull request #4166 from kampka/nix-conf-env

Add NIX_CONFIG env var for applying nix.conf overrides

doc/manual/src/command-ref/conf-file-prefix.md

@@ -19,6 +19,9 @@ By default Nix reads settings from the following places:
     and `XDG_CONFIG_HOME`. If these are unset, it will look in
     `$HOME/.config/nix.conf`.
 
+  - If `NIX_OPTIONS` is set, its contents is treated as the contents of
+    a configuration file.
+
 The configuration files consist of `name =
 value` pairs, one per line. Other files can be included with a line like
 `include

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

@@ -81,6 +81,11 @@ Most Nix commands interpret the following environment variables:
     Overrides the location of the system Nix configuration directory
     (default `prefix/etc/nix`).
 
+  - `NIX_OPTIONS`  
+    Applies settings from Nix configuration from the environment.
+    The content is treated as if it was read from a Nix configuration file.
+    Settings are separated by the newline character.
+
   - `NIX_USER_CONF_FILES`  
     Overrides the location of the user Nix configuration files to load
     from (defaults to the XDG spec locations). The variable is treated