Add JAVAC to the list of env variables to unset.

Some linux environments export JAVAC for legacy compatibility and this
 variable must be unset to ensure Soong and Make JAVAC values match.

Test: manual

Change-Id: Iae19fcb91b0a7651cbe00fe50f6daed54b4d7879
diff --git a/ui/build/config.go b/ui/build/config.go
index cb7fe1e..73e2c45 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -369,6 +369,7 @@
 		"CDPATH",
 		"DISPLAY",
 		"GREP_OPTIONS",
+		"JAVAC",
 		"NDK_ROOT",
 		"POSIXLY_CORRECT",