building xsnap depends on fragile git clone

[dckc]

to reproduce:

1. yarn create @agoric/dapp xsnapv
2. yarn
3. cd node_modules/@agoric/xsnap
4. ls .git and see that we have a git log of moddable

The git clone is fragile. One user recently reported:

Building...
 - @agoric/xsnap
...
The remote end hung up unexpectedly
...
git exited with code 128

This was fixed in

• build xsnap from NPM tarball without git agoric-sdk#8536

but this dapp still depends on 0.14.3-u13.0

so if I just go to a blank directory and yarn install @agoric/xsnap, I get @agoric/xsnap@0.14.3-u16.1 and I do not get a .git directory.

cc @Jovonni