build-bazel-package: switch hash mode to “flat”

flat hashes can be substituted through hashed-mirrors, while recursive
hashes can’t. This is especially important for Bazel since the bazel
fetch dependencies can come from multiple different methods (git,
http, ftp, etc.). To do this, we create tar archives from the
output/external directory, which is then extracted to build. All of
the Bazel hashes are all updated.