Fix logic error in prebuilt.py
Fix logic error in prebuilt.py

Introduced in Ic6b6f978e28be8cd3d8a4cf530d5333e3176b7c1, this
is a twisty bit of logic that basically ensures the file exists.  Thus
change the mode to append, preserving any pre-existing data.

BUG=None
TEST=None

Change-Id: Ia686f3db2949a2f80df5900b65fc749bc4b0f643
Reviewed-on: https://gerrit.chromium.org/gerrit/22532
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

chadversary/chromiumos.chromite,zhang0137/chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite