wheel: also build the musl wheel in the ci

We add a trigger to not run that all the time.

contrib/heptapod-ci.yml

@@ -127,6 +127,27 b' build-c-wheel:'
           - cp312-cp312
           - cp313-cp313
 
+trigger-wheel-musl:
+  extends: .trigger
+  stage: build
+  rules:
+  - if: $CI_COMMIT_BRANCH =~ $RE_BRANCH
+    when: never
+  - if: $CI_COMMIT_BRANCH =~ $RE_TOPIC
+    when: manual
+    allow_failure: true
+
+build-c-wheel-musl:
+  extends: build-c-wheel
+  image: "registry.heptapod.net/mercurial/ci-images/core-wheel-x86_64-musl-c:v3.0"
+  rules:
+  - if: $CI_COMMIT_BRANCH =~ $RE_BRANCH
+    needs:
+      - trigger-nightly-build
+  - if: $CI_COMMIT_BRANCH =~ $RE_TOPIC
+    needs:
+      - "trigger-wheel-musl"
+
 .runtests:
     extends: .all
     stage: tests
@@ -581,6 +602,7 b' upload-wheel-nightly:'
     - build-c-wheel
     - build-c-wheel-macos
     - build-c-wheel-windows
+    - build-c-wheel-musl
     - test-result-linux
     - test-result-macos
     - test-result-windows