element.py: Make artifact lowercase in time_activity()

Just a small bug-bear here.

I noticed that the build output would output:

• Caching Artifact
• Pushing Artifact
• Sending artifact

Note the difference in case.

To keep consistent with the rest of the build output (which only has the first word in uppercase) I've made this small change.

Here is an example build with this change implemented:

jamesennis@ct-lt-1030:~/bst_projects/gnome-build-meta$ bst build --track-all core-deps/yelp-tools.bst 

...some stuff...

[--:--:--][b9db1c83][build:core-deps/yelp-tools.bst  ] START   Caching artifact
[00:00:00][b9db1c83][build:core-deps/yelp-tools.bst  ] SUCCESS Caching artifact
[00:00:21][b9db1c83][build:core-deps/yelp-tools.bst  ] SUCCESS home/jamesennis/.cache/buildstream/logs/gnome/core-deps-yelp-tools/b9db1c83-build.19983.log
[--:--:--][b9db1c83][ push:core-deps/yelp-tools.bst  ] START   home/jamesennis/.cache/buildstream/logs/gnome/core-deps-yelp-tools/b9db1c83-push.21021.log
[--:--:--][b9db1c83][ push:core-deps/yelp-tools.bst  ] START   Pushing artifact
[--:--:--][b9db1c83][ push:core-deps/yelp-tools.bst  ] START   Preparing compressed archive
[00:00:00][b9db1c83][ push:core-deps/yelp-tools.bst  ] SUCCESS Preparing compressed archive
[--:--:--][b9db1c83][ push:core-deps/yelp-tools.bst  ] START   Sending artifact
[00:00:00][b9db1c83][ push:core-deps/yelp-tools.bst  ] SUCCESS Sending artifact
[00:00:00][b9db1c83][ push:core-deps/yelp-tools.bst  ] SUCCESS Pushing artifact
[00:00:00][b9db1c83][ push:core-deps/yelp-tools.bst  ] SUCCESS home/jamesennis/.cache/buildstream/logs/gnome/core-deps-yelp-tools/b9db1c83-push.21021.log
[00:00:24][][] SUCCESS Build Complete

Pipeline Summary
  Total:       6
  Session:     6
  Track Queue: processed 0, skipped 6, failed 0 
  Pull Queue:  processed 0, skipped 6, failed 0 
  Fetch Queue: processed 0, skipped 6, failed 0 
  Build Queue: processed 1, skipped 5, failed 0 
  Push Queue:  processed 1, skipped 5, failed 0