Add CCACHE_DIR to allowed environment variables

Trying to override the default directory for ccache by using the
CCACHE_DIR environment variable fails unless it is in the allowed
list.

Bug: 149670916
Test: manual
Change-Id: I8e7eea7a5c25d7ea5f0956fafc70d62522f3c4fc

ui/build/ninja.go

@@ -142,6 +142,7 @@ func runNinja(ctx Context, config Config) {
 			"CCACHE_SLOPPINESS",
 			"CCACHE_BASEDIR",
 			"CCACHE_CPP2",
+			"CCACHE_DIR",
 		}, config.BuildBrokenNinjaUsesEnvVars()...)...)
 	}