.arcconfig

tests: stabilize `test-clonebundles-autogen.t` on Windows

The problem was that the commands are spun up with `shell=True`, which uses
`cmd.exe`, which doesn't understand `$foo` style variables.  The HGCB variable
expansion has to be delayed, because it's figured out right before launching
the command.  We could probably add a conditional for Windows, and rewrite the
config to use `%foo%` style variables, but it's more maintainable to just wrap
the command in a bash shell invocation.

The forward style slashes in the path are needed to avoid accruing double
backslashes (when switching between shells- the url template seems fine).  Also
need to strong quote the command so that the double quotes don't get stripped
off of `$HGCB_BUNDLE_PATH`, which results in:

    sh: 1: Syntax error: Unterminated quoted string
    abort: command returned status 2: sh -c "cp $HGCB_BUNDLE_PATH $TESTTMP/final-upload/"

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}