brew.sh: enable linkage cache for developers.

This seems stable on CI so I think we can expose it to more people.

Library/Homebrew/brew.sh

@@ -277,6 +277,11 @@ then
   HOMEBREW_BASH_COMMAND="$HOMEBREW_LIBRARY/Homebrew/dev-cmd/$HOMEBREW_COMMAND.sh"
 fi
 
+if [[ -n "$HOMEBREW_DEVELOPER" || -n "$HOMEBREW_DEV_CMD_RUN" ]]
+then
+  export HOMEBREW_LINKAGE_CACHE="1"
+fi
+
 check-run-command-as-root() {
   [[ "$(id -u)" = 0 ]] || return
 

Library/Homebrew/dev-cmd/tests.rb

@@ -38,7 +38,6 @@ module Homebrew
       ENV.delete("VERBOSE")
       ENV.delete("HOMEBREW_CASK_OPTS")
       ENV.delete("HOMEBREW_TEMP")
-      ENV.delete("HOMEBREW_LINKAGE_CACHE")
       ENV.delete("HOMEBREW_NO_GITHUB_API")
       ENV.delete("HOMEBREW_NO_EMOJI")
       ENV["HOMEBREW_NO_ANALYTICS_THIS_RUN"] = "1"