debugger: use NO_COLOR to disable coloring

Do not use non-standard DBG_NOCOLOR, but use NO_COLOR [^1]
for disabling color output on terminals.

[^1]: https://no-color.org/

NO_DOC=later
NO_CHANGELOG=internal
NO_TEST=internal

third_party/lua/README-luadebug.md

@@ -110,7 +110,7 @@ There are also some goodies you can use to make debugging easier.
 Environment Variables:
 -
 
-Want to disable ANSI color support or disable GNU readline? Set the `DBG_NOCOLOR` and/or `DBG_NOREADLINE` environment variables.
+Want to disable ANSI color support or disable GNU readline? Set the `NO_COLOR`.
 
 Known Issues:
 -

third_party/lua/luadebug.lua

@@ -750,7 +750,7 @@ local stdout_isatty = ffi.C.isatty(1)
 
 -- Conditionally enable color support.
 local color_maybe_supported = (stdout_isatty and os.getenv("TERM") and os.getenv("TERM") ~= "dumb")
-if color_maybe_supported and not os.getenv("DBG_NOCOLOR") then
+if color_maybe_supported and not os.getenv("NO_COLOR") then
     COLOR_GRAY = string.char(27) .. "[90m"
     COLOR_RED = string.char(27) .. "[91m"
     COLOR_BLUE = string.char(27) .. "[94m"