sshfs example is broken

[imjasonh]

#143 adds e2e tests in CI that build all our examples, and seems to indicate the sshfs.yaml example fails at head:

https://github.com/chainguard-dev/melange/actions/runs/3299985902/jobs/5443974029

Running from head (in an alpine container with go and bubblewrap):

go run ./ build examples/sshfs.yaml --pipeline-dir=pipelines --arch=amd64
...
2022/10/21 19:41:03 melange (sshfs/x86_64): fatal: destination path '.' already exists and is not an empty directory.

[imjasonh]

Also mbedtls: https://github.com/chainguard-dev/melange/actions/runs/3300008753/jobs/5444023407

2022/10/21 19:40:49 melange (mbedtls/x86_64): fatal: destination path '.' already exists and is not an empty directory.

[imjasonh]

Seems these examples expect to be run with --empty-workspace, and fail otherwise since git-checkout's default destination is . (the workspace).

Running with --empty-workspace shows mbedtls still fails because the branch was renamed, but that can be fixed.

#143 should fix this if it can.