brew: set Bintray variables.

These are set as `HOMEBREW_BINTRAY_*` and then used by the rest of
`brew` as normal.

bin/brew

@@ -44,7 +44,7 @@ fi
 
 HOMEBREW_LIBRARY="$HOMEBREW_REPOSITORY/Library"
 
-for VAR in EDITOR PATH
+for VAR in EDITOR PATH BINTRAY_USER BINTRAY_KEY
 do
   VAR_NEW="HOMEBREW_${VAR}"
   [[ -n "${!VAR_NEW}" ]] && continue