ci: use RelWithDebInfo build for non-master branches

- unlike Release, it builds with debug assertions and eigen-nan
  intialization, which may help to spot some bugs earlier.

.gitlab-ci.yml

@@ -91,6 +91,8 @@ stages:
 bionic-release-compile:
   <<: *prepare_docker_definition
   <<: *compile_test_package_definition
+  only:
+    - master
   variables:
     CXX_MARCH: 'corei7-avx'
     DEB_DIR: deb_bionic
@@ -125,8 +127,6 @@ bionic-debug-compile:
 bionic-relwithdebinfo-compile:
   <<: *prepare_docker_definition
   <<: *compile_test_package_definition
-  only:
-    - master
   variables:
     BUILD_TYPE: RelWithDebInfo
 
@@ -160,6 +160,8 @@ clang-format:
 
 bionic-test:
   <<: *test_deb_definition
+  only:
+    - master
   variables:
     DEB_DIR: deb_bionic
     GIT_STRATEGY: none