charm build fails on flannel 'go build'

[Kevin W Monroe]

Sample failure:

https://jenkins.canonical.com/k8s/job/build-charms/763/consoleFull

```
01:30:45 [flannel] + echo 'Building etcd v3.4.22 for amd64'
01:30:45 [flannel] Building etcd v3.4.22 for amd64
01:30:45 [flannel] + docker run --rm -e GOOS=linux -e GOARCH=amd64 -v /var/lib/jenkins/slaves/jenkins-slave-focal-ps5-12/workspace/build-charms/.cache/charmbuild/jenkins-build-charms-763/charms/flannel/tmp/build-flannel-resources.tmp/etcd:/etcd golang:1.19 /bin/bash -c 'cd /etcd && ./build && chown -R 997:997 /etcd'
...
01:30:46 [flannel] go: downloading github.com/matttproud/golang_protobuf_extensions v1.0.1
01:30:47 [flannel] error obtaining VCS status: exit status 128
01:30:47 [flannel] Use -buildvcs=false to disable VCS stamping.
01:30:47 [flannel] Traceback (most recent call last):
01:30:48 File "jobs/build-charms/charms.py", line 1058, in build
01:30:48 entity.attach_resources()
01:30:48 File "jobs/build-charms/charms.py", line 837, in attach_resources
01:30:48 raise BuildException("Failed to build custom resources")
01:30:48 BuildException: Failed to build custom resources
01:30:48
01:30:48 [flannel] Stopping
```

Happens with go 1.18+ because git isn't in the docker build env, but VCS info is requested (go -buildvcs=true by default).

[Kevin W Monroe (kwmonroe)]

Fix will happen in the flannel charm repo, not jenkins. PR for review:

https://github.com/charmed-kubernetes/charm-flannel/pull/87