Revert "Switch to Python prebuilts."

This reverts commit b08dae5a85773f220e450d2f12be72039cd97374.

The mac builders are too old for this Python. Go back to bootstrapping
but pin the manifest so we're not holding back other branches.

Test: treehugger
Bug: None
Change-Id: I50fd76b1b116a436ab221436aeca21ac4b716c63
1 file changed
tree: 13410bb50840e0a476f07c57d364eee9ade7a036
  1. default.xml