Comment 3 for bug 2019864

I reproduced this by (main steps):

1. git ubuntu clone simplestreams
2. git checkout ubuntu/bionic-devel
3. sbuild -d bionic

Outcome:

[...]
make[1]: Entering directory '/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460'
./tools/create-gpgdir
creating GNUPGHOME dir in /build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/gnupg.
  pubkey '/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/examples/keys/example.pub'
  secret '/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/examples/keys/example.sec'
  pubkeys: /build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/examples/keys/example.pub
imported secret key /build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/examples/keys/example.sec
imported pubkey /build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/examples/keys/example.pub
./tools/tenv ./tools/sign-examples
Traceback (most recent call last):
  File "/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/tools/js2signed", line 53, in <module>
    main()
  File "/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/tools/js2signed", line 41, in main
    signjson_file(path, force=force)
  File "/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/tools/sign_helper.py", line 25, in signjson_file
    util.sign_file(fname, inline=False)
  File "/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/simplestreams/util.py", line 546, in sign_file
    return subp(get_sign_cmd(path=fname, output=outfile, inline=inline))[0]
  File "/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/simplestreams/util.py", line 458, in subp
    raise subprocess.CalledProcessError(rc, args, output=(out, err))
subprocess.CalledProcessError: Command '['gpg', '--batch', '--output', '/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/examples/cirros/streams/v1/index.json.gpg', '--armor', '--detach-sign', '/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460/examples/cirros/streams/v1/index.json']' returned non-zero exit status 2
Makefile:39: recipe for target 'examples-sign' failed
make[1]: *** [examples-sign] Error 1
make[1]: Leaving directory '/build/simplestreams-83uOzI/simplestreams-0.1.0~bzr460'
dh_auto_test: make -j1 test returned exit code 2
debian/rules:7: recipe for target 'build' failed
make: *** [build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2
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