cibuild.sh: Create $prebuilt/env.sh

It aimed to be sourced by user shell to update PATH.
diff --git a/cibuild.sh b/cibuild.sh
index 508c43f..d766780 100755
--- a/cibuild.sh
+++ b/cibuild.sh
@@ -31,6 +31,7 @@
 tools=$WD/../tools
 prebuilt=$WD/../prebuilt
 os=$(uname -s)
+EXTRA_PATH=
 
 case $os in
   Darwin)
@@ -45,6 +46,7 @@
 
 function add_path {
   PATH=$1:$PATH
+  EXTRA_PATH=$1:$EXTRA_PATH
 }
 
 function python-tools {
@@ -281,6 +283,8 @@
     $func
   done
   popd
+
+  echo PATH=$EXTRA_PATH\$PATH > $prebuilt/env.sh
 }
 
 function run_builds {