Nix build fails for headplane-ssh-wasm after DERP port patch

[eccgecko]

Description

The Nix build of headplane-ssh-wasm fails after commit 44dffea.
Two bugs in nix/ssh-wasm.nix:

1. Wrong patch directory (-d vendor should be -d vendor/tailscale.com)
2. Vendor files are read-only in the Nix sandbox (needs chmod before patch)

Fix: #522

Headplane Version

HEAD

Headscale Version

v0.28.0