cerbero: Run download tool outside of the build env
When we run from inside the build env, LD_LIBRARY_PATH is set to point
to the cerbero prefix, which contains an old version of GnuTLS. This
is no longer possible in master because we no longer set the build env
in os.environ
there. Fixes:
wget: [...]/dist/linux_x86_64/lib/libgnutls.so.30: version `GNUTLS_3_6_3' not found (required by wget)