Fix TMUX env variable filtering

We currently filter out `TMUX`, but this breaks displaying some caveats.
This also enables an alias I use (and improved by @Rylan12):

    brew alias fzp='!id="$(gh pr list -L200 -R github.com/Homebrew/homebrew-core | TMUX=$HOMEBREW_TMUX fzf-tmux -p "90%,50%" --multi | cut -f1)"; [ -n "$id" ] && brew pr-publish --autosquash $id'

Library/Homebrew/extend/os/mac/caveats.rb

@@ -48,7 +48,7 @@ class Caveats
 
     # pbpaste is the system clipboard tool on macOS and fails with `tmux` by default
     # check if this is being run under `tmux` to avoid failing
-    if ENV["TMUX"] && !quiet_system("/usr/bin/pbpaste")
+    if ENV["HOMEBREW_TMUX"] && !quiet_system("/usr/bin/pbpaste")
       s << "" << "WARNING: brew services will fail when run under tmux."
     end
     "#{s.join("\n")}\n" unless s.empty?

Library/Homebrew/test/caveats_spec.rb

@@ -135,7 +135,7 @@ describe Caveats do
             "plist_test.plist"
           end
         end
-        ENV["TMUX"] = "1"
+        ENV["HOMEBREW_TMUX"] = "1"
         allow(Homebrew).to receive(:_system).with("/usr/bin/pbpaste").and_return(false)
         caveats = described_class.new(f).caveats
 

bin/brew

@@ -64,7 +64,7 @@ HOMEBREW_LIBRARY="${HOMEBREW_REPOSITORY}/Library"
 
 # Copy and export all HOMEBREW_* variables previously mentioned in
 # manpage or used elsewhere by Homebrew.
-for VAR in BROWSER DISPLAY EDITOR NO_COLOR PATH
+for VAR in BROWSER DISPLAY EDITOR NO_COLOR PATH TMUX
 do
   # Skip if variable value is empty.
   [[ -z "${!VAR}" ]] && continue