Give CI its own SCCACHE_SERVER_PORT

Sharing the default port meant whichever context's server happened to
be running (usually the interactive shell's, via rust-analyzer's
background cargo checks) silently served the other's build requests
too - and since sccache's server writes outputs under its own user,
that meant permission-denied for whichever context didn't own it.
Separate ports keep the server processes apart; SCCACHE_DIR is what's
actually shared, giving real cache hits across both.

.gitea/workflows/deploy.yml

@@ -18,7 +18,16 @@ jobs:
       # Shared with interactive dev builds (see ~/.config/fish/config.fish)
       # so a crate compiled once, by either a manual build or CI, is
       # cached for the other - real cache hits, not just a warm toolchain.
+      # SCCACHE_SERVER_PORT deliberately differs from the interactive
+      # dev shell's (4227): sccache's server is discovered by a fixed
+      # TCP port shared by every local user, so if both contexts used
+      # the same port, whichever one's server happened to be running
+      # would silently "win" and serve build requests it doesn't have
+      # filesystem permission for. Separate ports keep each context's
+      # own server answering its own requests; the cache directory
+      # (not the server process) is what's actually shared.
       SCCACHE_DIR: /var/local/sccache
+      SCCACHE_SERVER_PORT: "4228"
       CARGO_TARGET_DIR: /var/local/cargo-target
     steps:
       - uses: actions/checkout@v4