ci: drop wrongly calling make install-lint

Linting tools should already be installed by Nix, and we should use
them.

Also switching most shells to pure mode to see what happens.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

_assets/ci/Jenkinsfile.tests

@@ -57,14 +57,13 @@ pipeline {
 
     stage('Lint') {
       steps { script {
-        nix.shell('make install-lint', pure: false)
-        nix.shell('make lint', pure: false)
+        nix.shell('make lint', pure: true)
       } }
     }
 
     stage('Canary') {
       steps { script {
-        nix.shell('make canary-test', pure: false)
+        nix.shell('make canary-test', pure: true)
       } }
     }