ci: avoid sudo requirement for self-hosted cirrus

.cirrus.yml

@@ -45,6 +45,9 @@ env:  # Global defaults
 # - medium: For an x86_64 machine, recommended to have 4 CPUs and 16 GB of memory.
 # - noble: For a machine running the Linux kernel shipped with exaclty Ubuntu Noble 24.04. The machine is recommended to have 4 CPUs and 16 GB of memory.
 # - arm64: For an aarch64 machine, recommended to have 2 CPUs and 8 GB of memory.
+#
+# For works without sudo rights, start them with USER_MODE=true and make sure:
+# - git is installed
 
 # https://cirrus-ci.org/guide/tips-and-tricks/#sharing-configuration-between-tasks
 filter_template: &FILTER_TEMPLATE
@@ -60,7 +63,8 @@ base_template: &BASE_TEMPLATE
   << : *FILTER_TEMPLATE
   merge_base_script:
     # Unconditionally install git (used in fingerprint_script).
-    - bash -c "$PACKAGE_MANAGER_INSTALL git"
+    # In USER_MODE this must have been done manually
+    - if [ ! "$USER_MODE" = "true" ]; then bash -c "$PACKAGE_MANAGER_INSTALL git"; fi
     - if [ "$CIRRUS_PR" = "" ]; then exit 0; fi
     - git fetch --depth=1 $CIRRUS_REPO_CLONE_URL "pull/${CIRRUS_PR}/merge"
     - git checkout FETCH_HEAD  # Use merged changes to detect silent merge conflicts