ci: install fuse3 when runners lack fusermount3 (#12013)

Self-hosted runners do not ship fuse3, so the setuid repair step
failed with 'no fusermount3 in PATH' before the FUSE jobs could run.
Install the package instead of failing outright.

.github/actions/fix-fusermount-setuid/action.yml

@@ -23,8 +23,14 @@ runs:
 
         bin=$(command -v fusermount3 || true)
         if [ -z "$bin" ]; then
-          echo "no fusermount3 in PATH" >&2
-          exit 1
+          # Self-hosted runners are not guaranteed to carry fuse3.
+          sudo apt-get update
+          sudo apt-get install -y fuse3
+          bin=$(command -v fusermount3 || true)
+          if [ -z "$bin" ]; then
+            echo "no fusermount3 in PATH after installing fuse3" >&2
+            exit 1
+          fi
         fi
         if can_mount "$bin"; then
           ls -l "$bin"