shell.py: when wget check certificates it doesn't use the system library for them

Submitted by vin..@..il.com

Description

In the latest Ubuntu 18.04 LTS Cerbero fails to build gstreamer-1.0, since wget check the certificates, but in the build environment the ones hosted in the building systems are not reachable.

The error is like:

[(65/80) spandsp -> fetch ]
-----> Fetching tarball
http://www.soft-switch.org/downloads/spandsp/spandsp-0.0.6.tar.gz to /vol2/tdprime/cerbero/build/sources/local/spandsp-0.0.6/spandsp-0.0.6.tar.gz
Downloading
http://www.soft-switch.org/downloads/spandsp/spandsp-0.0.6.tar.gz
Running command 'wget
http://www.soft-switch.org/downloads/spandsp/spandsp-0.0.6.tar.gz -O /vol2/tdprime/cerbero/build/sources/local/spandsp-0.0.6/spandsp-0.0.6.tar.gz
'
URL transformed to HTTPS due to an HSTS policy
--2018-05-18 23:12:50--
https://www.soft-switch.org/downloads/spandsp/spandsp-0.0.6.tar.gz
Resolving www.soft-switch.org (www.soft-switch.org)... 209.105.235.30 Connecting to www.soft-switch.org (www.soft-switch.org)|209.105.235.30|:443... connected.
ERROR: cannot verify www.soft-switch.org's certificate, issued by ‘CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US’:
Unable to locally verify the issuer's authority.
To connect to www.soft-switch.org insecurely, use `--no-check-certificate'.

Several packages seems affected, and not downloadable.