nightly: replace / from fingerprint in SSH key filename, fixes #423

The SSH key fingerprint is used in the filename.  The base64 used for SSH
key fingerprints includes /.  Not all keys will end up having a / in them.
For those that do, this will crash since the ssh key filename ends up being
non-existent dirs:

$ fdroid nightly
Importing keystore /home/mhoffmann/.android/debug.keystore to /tmp/.cqswaeo8/.keystore.p12...
MAC verified OK
writing RSA key
CRITICAL: Unknown exception found!
Traceback (most recent call last):
  File "/usr/lib/python3.6/shutil.py", line 544, in move
    os.rename(src, real_dst)
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/.cqswaeo8/.privkey' -> '/tmp/.cqswaeo8/debug_keystore_PZtS/4Tzk4dpzKiX9AAf1GrhAVi9U7UE1aYEHr6evKo_id_rsa'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/mhoffmann/projects/oss/fdroidserver/fdroid", line 156, in <module>
    main()
  File "/home/mhoffmann/projects/oss/fdroidserver/fdroid", line 132, in main
    mod.main()
  File "/home/mhoffmann/projects/oss/fdroidserver/fdroidserver/nightly.py", line 284, in main
    privkey = _ssh_key_from_debug_keystore()
  File "/home/mhoffmann/projects/oss/fdroidserver/fdroidserver/nightly.py", line 73, in _ssh_key_from_debug_keystore
    shutil.move(privkey, ssh_private_key_file)
  File "/usr/lib/python3.6/shutil.py", line 558, in move
    copy_function(src, real_dst)
  File "/usr/lib/python3.6/shutil.py", line 257, in copy2
    copyfile(src, dst, follow_symlinks=follow_symlinks)
  File "/usr/lib/python3.6/shutil.py", line 121, in copyfile
    with open(dst, 'wb') as fdst:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/.cqswaeo8/debug_keystore_PZtS/4Tzk4dpzKiX9AAf1GrhAVi9U7UE1aYEHr6evKo_id_rsa'
2 jobs for more-nightly in 28 minutes and 9 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #43666381
metadata_v0

00:03:52

passed #43666380
test

00:28:09