The release script assumes that the remote for the vmware-tanzu/velero
repository is called `upstream`. It may be the case that this remote is
configured to use a different name. This change updates the script to
allow the remote name being used to be configured by setting the
environment variable `REMOTE` before running the script. If the variable
is not set, the remote defaults to `upstream`.
The release instructions have also been updated to reflect this change.
Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>