Merge style checks into tests

They're both run in pretty much the same environment, and it's
unnecessary to spin up a separate VM for that.

Author: Hugo Osvaldo Barrera

.builds/style.yaml

@@ -16,10 +16,14 @@ tasks:
       cd vdirsyncer
       sudo systemctl start docker
       DAV_SERVER="radicale xandikos" make -e install-test
+      make -e install-style
   - test: |
       cd vdirsyncer
       # Non-system python is used for packages:
       export PATH=$PATH:~/.local/bin/
       make -e ci-test
       DAV_SERVER=radicale make -e ci-test-storage
       DAV_SERVER=xandikos make -e ci-test-storage
+  - style: |
+      make -e install-style
+      make -e style