.github: set user_allow_other in /etc/fuse.conf

Fixes this test failure:

2023/04/13 20:25:20 callFusermount: executing ["/usr/bin/fusermount3" "/tmp/TestDirectMount4166040800" "-o" "allow_other,subtype=/tmp/go-build1065896,max_read=131072"]
/usr/bin/fusermount3: option allow_other only allowed if 'user_allow_other' is set in /etc/fuse.conf

\--- FAIL: TestDirectMount (0.01s)
    mount_linux_test.go:117: fusermount exited with code 256

Change-Id: I4a2a2983dd422de9360b17068ac0d8a3947d453d

.github/workflows/ci.yml

@@ -31,6 +31,7 @@ jobs:
 
     # CI platform specific setup steps happen here
     - run: sudo apt-get install -qq fuse3 libssl-dev libfuse-dev
+    - run: echo user_allow_other | sudo tee -a /etc/fuse.conf
 
     # Actual test steps are in all.bash
     - run: ./all.bash