fix(build): make the local + CI Linux build work off main (#26)

Three independent gaps currently break a from-scratch Linux build (and CI):

1. .build-config.json is missing `versions.dotnet`, yet both the devcontainer
   Dockerfile (`dotnet-install.sh --channel ${DOTNET_VERSION}`) and the CI
   workflows (`require('./.build-config.json').versions.dotnet` → setup-dotnet
   `${dotnet}.x`) read it. Empty → the image build / setup-dotnet step fails.
   Add `"dotnet": "8.0"` (Velopack's vpk targets .NET 8; "8.0" satisfies both
   `--channel 8.0` and setup-dotnet's `8.0.x`). Keeps .NET available for
   `vpk pack` on tagged release builds.

2. build-linux-docker.sh never forwards GH_CLONE_TOKEN or SLOPSMITH_REF into
   the container, so a local `bash scripts/build-linux-docker.sh` can't clone
   the private got-feedback core/plugins or select a core ref. Pass both
   through (defaulting SLOPSMITH_REF to main).

3. The ffmpeg libvorbis guard pipes `ffmpeg | grep -wq`, which is racy under
   `set -o pipefail`: grep -q closes the pipe on first match, ffmpeg takes
   SIGPIPE (141), pipefail returns 141, and the guard false-fails even though
   libvorbis is present (reproducible on Docker Desktop). Capture the encoder
   list into a variable first, then grep.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.build-config.json

@@ -4,7 +4,8 @@
     "python": "3.12.9",
     "electron": "35.0.0",
     "cmake": "3.22",
-    "ubuntu": "22.04"
+    "ubuntu": "22.04",
+    "dotnet": "8.0"
   },
   "external": {
     "fluidsynth_windows": {