ci(audience): force StandaloneLinux64 player to OpenGLCore only at build time

Adds GraphicsApisLinuxOverride, an editor build hook gated on the
AUDIENCE_LINUX_GLCORE_ONLY env var. When the env is set and the build
target is StandaloneLinux64, it pins PlayerSettings graphics APIs to
OpenGLCore only, dropping Vulkan from the active list.

The Linux container has no GPU and the player runs on Mesa llvmpipe via
-force-glcore. Vulkan was active in the build only because Unity's
default Standalone API list includes it; the shader compiler emitted
both glcore and vulkan variants for every shader. On the Unity 6 cell
this was 213 of 413 compiles wasted on a code path the player never
hit.

Wires the env var through the playmode-linux job's docker run so the
override fires on Linux PlayMode CI builds. Local builds and other
targets see no change because the env var is unset and the hook short
circuits on non-Linux build targets.

Author: ImmutableJeffrey

.github/workflows/test-audience-sample-app.yml

@@ -467,6 +467,7 @@ jobs:
           AUDIENCE_TEST_PUBLISHABLE_KEY: ${{ secrets.AUDIENCE_TEST_PUBLISHABLE_KEY }}
           AUDIENCE_SCRIPTING_BACKEND: ${{ matrix.backend }}
           UNITY_VERSION: ${{ matrix.unity }}
+          AUDIENCE_LINUX_GLCORE_ONLY: "1"
         run: |
           set -uo pipefail
           mkdir -p artifacts
@@ -482,6 +483,7 @@ jobs:
             --env UNITY_EMAIL --env UNITY_PASSWORD --env UNITY_SERIAL \
             --env AUDIENCE_TEST_PUBLISHABLE_KEY --env AUDIENCE_SCRIPTING_BACKEND \
             --env AUDIENCE_TEST_RUN_ID --env AUDIENCE_TEST_CELL_ID \
+            --env AUDIENCE_LINUX_GLCORE_ONLY \
             --volume "$PWD":/github/workspace:z \
             --cpus=8 --memory=30487m \
             "$image" \

examples/audience/Assets/Editor/GraphicsApisLinuxOverride.cs

@@ -0,0 +1,56 @@
+#nullable enable
+
+using System;
+using UnityEditor;
+using UnityEditor.Build;
+using UnityEditor.Build.Reporting;
+using UnityEngine;
+using UnityEngine.Rendering;
+
+namespace Immutable.Audience.Samples.SampleApp.Editor
+{
+    // Build pre-process hook that restricts the StandaloneLinux64 player to
+    // OpenGLCore when AUDIENCE_LINUX_GLCORE_ONLY is set.
+    //
+    // Why: the unityci/editor Linux container has no GPU. Unity falls back
+    // to Mesa software OpenGL via llvmpipe. The runtime -force-glcore flag
+    // picks OpenGL when the player launches, but the build still ships
+    // every active graphics API's shader variants. With Vulkan also active
+    // by default, the shader compiler emits both glcore and vulkan
+    // variants. Roughly half of the 413 shader compiles measured on Unity
+    // 6 Linux were wasted on Vulkan variants the player never used.
+    //
+    // The hook runs only when the env flag is set, only for the
+    // StandaloneLinux64 build target, and only modifies in-memory
+    // PlayerSettings during the build. Other Standalone targets (Win, Mac)
+    // and other CI workflows are unaffected. Local builds without the env
+    // var see no change.
+    //
+    // Usage:
+    //   AUDIENCE_LINUX_GLCORE_ONLY=1 Unity -batchmode -buildTarget StandaloneLinux64 -runTests ...
+    internal sealed class GraphicsApisLinuxOverride : IPreprocessBuildWithReport
+    {
+        private const string EnvVar = "AUDIENCE_LINUX_GLCORE_ONLY";
+
+        public int callbackOrder => 1;
+
+        public void OnPreprocessBuild(BuildReport report)
+        {
+            if (report.summary.platform != BuildTarget.StandaloneLinux64) return;
+
+            var requested = Environment.GetEnvironmentVariable(EnvVar);
+            if (string.IsNullOrEmpty(requested)) return;
+
+            var current = PlayerSettings.GetGraphicsAPIs(BuildTarget.StandaloneLinux64);
+            if (current.Length == 1 && current[0] == GraphicsDeviceType.OpenGLCore)
+            {
+                Debug.Log($"[{nameof(GraphicsApisLinuxOverride)}] StandaloneLinux64 already at OpenGLCore only.");
+                return;
+            }
+
+            PlayerSettings.SetUseDefaultGraphicsAPIs(BuildTarget.StandaloneLinux64, false);
+            PlayerSettings.SetGraphicsAPIs(BuildTarget.StandaloneLinux64, new[] { GraphicsDeviceType.OpenGLCore });
+            Debug.Log($"[{nameof(GraphicsApisLinuxOverride)}] StandaloneLinux64 graphics APIs forced to OpenGLCore. Vulkan shader variants will be skipped.");
+        }
+    }
+}