Use /bin/bash for update_depot_tools.
Use /bin/bash for update_depot_tools.

On at least one platform /usr/bin/env is actually not
a guaranteed method to get to bash.  Fall back to using
/bin/bash for this script.
Review URL: http://codereview.chromium.org/7039045

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@86185 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools