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.

.builds/style.yaml

@@ -1,16 +0,0 @@
-image: archlinux
-packages:
-  - docker
-  - docker-compose
-  - python-pip
-sources:
-  - https://github.com/pimutils/vdirsyncer
-tasks:
-  - setup: |
-      cd vdirsyncer
-      make -e install-style
-  - test: |
-      cd vdirsyncer
-      # Non-system python is used for packages:
-      export PATH=$PATH:~/.local/bin/
-      make -e style

.builds/tests-release.yaml

@@ -16,6 +16,7 @@ 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:
@@ -23,3 +24,6 @@ tasks:
       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