Env/deps does not respect non-interactive mode

[mkowalski]

As an operator I want to have a possibility to perform a fully non-interactive installation of SCION and it's dependencies. Currently one of the steps is running ./env/deps script. This one requires an admin interaction as in the session below

scionlab@scionlab-1107-einstein:~/go/src/github.com/scionproto/scion$ ./env/deps 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  libsodium18 python3-cffi python3-cffi-backend python3-cryptography python3-idna python3-ply python3-pyasn1
  python3-pycparser
Suggested packages:
  python-cryptography-doc python3-cryptography-vectors python-nacl-doc python-openssl-doc python3-openssl-dbg python-ply-doc
  doc-base
The following NEW packages will be installed:
  libsodium18 python3-cffi python3-cffi-backend python3-cryptography python3-idna python3-nacl python3-openssl python3-ply
  python3-pyasn1 python3-pycparser
0 upgraded, 10 newly installed, 0 to remove and 109 not upgraded.
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As a result, installation or upgrade of SCION requires a manual interaction what is not a desired behaviour when performing the action in >1 machine.

[kormat]

The recommend approach for this is to do APTARGS=-y ./env/deps, which is already supported.