automatic retry on wget failure

[bmc-msft]

aka.ms is having stability issues. We've seen multiple build failures due to:

$ wget -O azcopy.zip https://aka.ms/downloadazcopy-v10-windows
--2021-03-10 19:03:54-- https://aka.ms/downloadazcopy-v10-windows
Resolving aka.ms (aka.ms)... 23.223.13.99
Connecting to aka.ms (aka.ms)|23.223.13.99|:443... connected.
OpenSSL: error:14094438:SSL routines:ssl3_read_bytes:tlsv1 alert internal error
Unable to establish SSL connection.
$

This PR should help by making wget automatically retry on failure. This will try up to 30 times, waiting 5 seconds between each attempt.