tox: Add skip_install to more environments.

tox.ini

@@ -61,6 +61,7 @@ deps = pep257==0.5.0
 commands = {envpython} -m pep257 scripts qutebrowser --ignore=D102,D209,D402 '--match=(?!resources|test_content_disposition).*\.py'
 
 [testenv:flake8]
+skip_install = true
 deps =
     -rrequirements.txt
     pyflakes==0.8.1
@@ -71,6 +72,7 @@ commands =
     {envdir}/bin/flake8 scripts qutebrowser --config=.flake8
 
 [testenv:pyroma]
+skip_install = true
 deps =
     pyroma==1.7
     docutils==0.12
@@ -79,6 +81,7 @@ commands =
     {envdir}/bin/pyroma .
 
 [testenv:check-manifest]
+skip_install = true
 deps =
     check-manifest==0.24
 commands =
@@ -86,6 +89,7 @@ commands =
     {envdir}/bin/check-manifest --ignore 'qutebrowser/git-commit-id,qutebrowser/html/doc,qutebrowser/html/doc/*,*/__pycache__'
 
 [testenv:docs]
+skip_install = true
 deps =
     -rrequirements.txt
 commands =