envsetup.sh: Fix the `reposync` function

It seems ionice can't find `repo` if it's in ~/bin, even if ~/bin
is in the user's $PATH.  Placing repo into /usr/bin or /usr/local/
bin works, but is undesirable (build instructions from both AOSP
and CM tell users to put repo into ~/bin, and repo likes to update
itself from time to time).

This forces the reposync function to use the full path to the repo
binary.

Change-Id: I9dc4a3d4ec3f39563e3a84de7321189700079c2e
diff --git a/envsetup.sh b/envsetup.sh
index c6fae0a..bd22cfa 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -1919,7 +1919,7 @@
             repo sync -j 4 "$@"
             ;;
         *)
-            schedtool -B -n 1 -e ionice -n 1 repo sync -j 4 "$@"
+            schedtool -B -n 1 -e ionice -n 1 `which repo` sync -j 4 "$@"
             ;;
     esac
 }