Re-add `HOMEBREW_DEBUG` environment variable.

Library/Homebrew/env_config.rb

@@ -268,6 +268,10 @@ module Homebrew
         description: "If set, always assume `--verbose` when running commands.",
         boolean:     true,
       },
+      HOMEBREW_DEBUG:                     {
+        description: "If set, always assume `--debug` when running commands.",
+        boolean:     true,
+      },
       HOMEBREW_VERBOSE_USING_DOTS:        {
         description: "If set, verbose output will print a `.` no more than once a minute. This can be " \
                      "useful to avoid long-running Homebrew commands being killed due to no output.",

docs/Manpage.md

@@ -1675,6 +1675,9 @@ For example, you might add something like the following to your ~/.profile, ~/.b
   * `HOMEBREW_VERBOSE`:
     If set, always assume `--verbose` when running commands.
 
+  * `HOMEBREW_DEBUG`:
+    If set, always assume `--debug` when running commands.
+
   * `HOMEBREW_VERBOSE_USING_DOTS`:
     If set, verbose output will print a `.` no more than once a minute. This can be useful to avoid long-running Homebrew commands being killed due to no output.
 

manpages/brew.1

 .\" generated with Ronn/v0.7.3
 .\" http://github.com/rtomayko/ronn/tree/0.7.3
 .
-.TH "BREW" "1" "August 2020" "Homebrew" "brew"
+.TH "BREW" "1" "September 2020" "Homebrew" "brew"
 .
 .SH "NAME"
 \fBbrew\fR \- The Missing Package Manager for macOS
@@ -2295,6 +2295,10 @@ If set, always use the latest stable tag (even if developer commands have been r
 If set, always assume \fB\-\-verbose\fR when running commands\.
 .
 .TP
+\fBHOMEBREW_DEBUG\fR
+If set, always assume \fB\-\-debug\fR when running commands\.
+.
+.TP
 \fBHOMEBREW_VERBOSE_USING_DOTS\fR
 If set, verbose output will print a \fB\.\fR no more than once a minute\. This can be useful to avoid long\-running Homebrew commands being killed due to no output\.
 .