Install curl before trying to call it

.gitlab-ci.yml

 image: docker:git
 
 services:
-- docker:dind
+  - docker:dind
 
 stages:
-- build
-- test
-- release
+  - build
+  - test
+  - release
 
 variables:
   TEST_IMAGE: briar/ci-image-android:${CI_BUILD_REF_NAME}
@@ -38,7 +38,9 @@ release:
 
 rebuild consumers:
   stage: release
+  before_script:
+    - apk add --update curl && rm -rf /var/cache/apk/*
   script:
-    - "curl -X POST -F token=${REBUILD_EMULATOR} -F ref=master -F variables[RELEASE_TAG]=${CI_COMMIT_REF_NAME} https://code.briarproject.org/api/v4/projects/225/trigger/pipeline"
+    - "curl -X POST -F token=${REBUILD_EMULATOR} -F ref=main -F variables[RELEASE_TAG]=${CI_COMMIT_REF_NAME} https://code.briarproject.org/api/v4/projects/225/trigger/pipeline"
   only:
     - master