github: Set GOTRACEBACK=crash on system test runs. (#14504)

Sets `GOTRACEBACK=crash` as a follow up to #14499 because in
@mihalicyn's words

> yeah, as withoutGOTRACEBACK="crash" we won't get a coredump at all, as
golang runtime handles segfaults gracefully and Linux kernel won't
realize that coredump collecting is required.

.github/workflows/tests.yml

@@ -209,6 +209,7 @@ jobs:
       LXD_VERBOSE: "1"
       LXD_OFFLINE: "1"
       LXD_TMPFS: "1"
+      GOTRACEBACK: "crash"
     name: System
     runs-on: ubuntu-22.04
     needs: code-tests
@@ -269,7 +270,7 @@ jobs:
           chmod +x ~
           echo "root:1000000:1000000000" | sudo tee /etc/subuid /etc/subgid
           cd test
-          sudo --preserve-env=PATH,GOPATH,GOCOVERDIR,GITHUB_ACTIONS,LXD_VERBOSE,LXD_BACKEND,LXD_CEPH_CLUSTER,LXD_CEPH_CEPHFS,LXD_CEPH_CEPHOBJECT_RADOSGW,LXD_OFFLINE,LXD_SKIP_TESTS,LXD_REQUIRED_TESTS, LXD_BACKEND=${{ matrix.backend }} ./main.sh ${{ matrix.suite }}
+          sudo --preserve-env=PATH,GOPATH,GOCOVERDIR,GITHUB_ACTIONS,LXD_VERBOSE,LXD_BACKEND,LXD_CEPH_CLUSTER,LXD_CEPH_CEPHFS,LXD_CEPH_CEPHOBJECT_RADOSGW,LXD_OFFLINE,LXD_SKIP_TESTS,LXD_REQUIRED_TESTS,GOTRACEBACK, LXD_BACKEND=${{ matrix.backend }} ./main.sh ${{ matrix.suite }}
 
       - name: Upload crash dumps
         if: always()