home-manager: respect NO_COLOR environment variable

This makes Home Manager respect the NO_COLOR environment variable to
disable coloring from output generated by Home Manager.

This initiative can be found more on https://no-color.org/

doc/release-notes/rl-2105.adoc

@@ -111,6 +111,9 @@ configure.packages.*.start  -> programs.neovim.plugins = [ { plugin = ...; }]
 configure.customRC -> programs.neovim.extraConfig
 ----
 
+* Home Manager now respects the `NO_COLOR` environment variable as per
+https://no-color.org/[].
+
 [[sec-release-21.05-state-version-changes]]
 === State Version Changes
 

home-manager/home-manager

@@ -221,7 +221,7 @@ function doSwitch() {
 function doListGens() {
     # Whether to colorize the generations output.
     local color="never"
-    if [[ -t 1 ]]; then
+    if [[ ! -v NO_COLOR && -t 1 ]]; then
         color="always"
     fi
 

modules/lib-bash/color-echo.sh

@@ -1,5 +1,7 @@
 # The check for terminal output and color support is heavily inspired
 # by https://unix.stackexchange.com/a/10065.
+#
+# Allow opt out by respecting the `NO_COLOR` environment variable.
 
 function setupColors() {
     normalColor=""
@@ -7,8 +9,8 @@ function setupColors() {
     warnColor=""
     noteColor=""
 
-    # Check if stdout is a terminal.
-    if [[ -t 1 ]]; then
+    # Enable colors for terminals, and allow opting out.
+    if [[ ! -v NO_COLOR && -t 1 ]]; then
         # See if it supports colors.
         local ncolors
         ncolors=$(tput colors)

modules/misc/news.nix

@@ -1873,6 +1873,13 @@ in
           'accounts.email.accounts.<name>.msmtp.enable' is true.
         '';
       }
+      {
+        time = "2021-03-03T22:16:05+00:00";
+        message = ''
+          Home Manager now respects the 'NO_COLOR' environment variable as per
+          https://no-color.org/.
+        '';
+      }
     ];
   };
 }