bootstrap.sh: Retry up to 3 times for transient download errors

[past-due]

From curl.1 the man page:

--retry <num>

If a transient error is returned when curl tries to perform a transfer, it will retry this number of times before giving up. Setting the number to 0 makes curl do no retries (which is the default). Transient error means either: a timeout, an FTP 4xx response code or an HTTP 408 or 5xx response code.

[ras0219-msft]

LGTM, thanks!