tox: Fix with-sslib-master install

pip nowadays recognizes that we are asking for two different versions of
securesystemslib in the "with-sslib-master" env, and errors out.

Instead install normal dependencies first, then install the new
securesystemslib separately (this ends up upgrading securesystemslib).

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tox.ini

@@ -29,10 +29,8 @@ install_command = python3 -m pip install {opts} {packages}
 # Develop test env to run tests against securesystemslib's master branch
 # Must to be invoked explicitly with, e.g. `tox -e with-sslib-master`
 [testenv:with-sslib-master]
-deps =
-    git+https://github.com/secure-systems-lab/securesystemslib.git@master#egg=securesystemslib[crypto,pynacl]
-    -r{toxinidir}/requirements-test.txt
-    --editable {toxinidir}
+commands_pre =
+    python3 -m pip install git+https://github.com/secure-systems-lab/securesystemslib.git@master#egg=securesystemslib[crypto,pynacl]
 
 commands =
     python3 -m coverage run aggregate_tests.py