Fix rogue cert directories causing EISDIR on start

[AaronFeledy]

Fixes #486

Docker creates missing bind-mount sources as directories. The v3 lando service bind-mounts ~/.lando/certs/<id>.<project>.crt|key whenever ssl: true, but those files are only generated at pre-start, so if Docker ever touches those mounts while the files are missing it leaves rogue directories behind — and cert generation then dies with EISDIR on every subsequent lando start/rebuild.

The first commits add a deterministic repro to the certs leia spec (rogue dirs at the cert paths before lando start), which failed CI with the exact error from the issue (failing run):

ERROR ==> EISDIR: illegal operation on a directory, open '/home/runner/.lando/certs/web.landocerts.crt'

Then the fix, in two layers:

• Recovery — utils/write-file.js now removes a directory sitting at the target path before writing. Existing files are never removed so bind-mounted files keep their inode across rewrites, preserving the Proxy - error mounting /certs/cert.key #242 WSL fix. Unit tests cover both behaviors.
• Prevention — builders/_lando.js ensures the cert paths exist as (possibly empty) files before they are ever bind-mounted, so Docker can no longer fabricate directories there in the first place.

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