build: Do not enable ASAN for musl CI pipelines

musl-libc does not currently support ASAN, so enabling it makes
configure fail to detect libmd as it cannot link to it anymore.

.gitlab-ci.yml

@@ -1,4 +1,9 @@
 .unit-tests:
+  script:
+    - ./autogen && ./configure
+    - make check
+
+.unit-tests-asan:
   script:
     - ./autogen && ./configure --enable-sanitize
     - make check
@@ -25,7 +30,7 @@
               bsd-compat-headers linux-headers libmd-dev musl-dev gcovr
 
 unit-tests:debian:
-  extends: [.debian, .unit-tests]
+  extends: [.debian, .unit-tests-asan]
 
 coverage:debian:
   extends: [.debian, .coverage]