ci(e2e): push test runner builds to Cachix

E2E Suites set up Nix without a Cachix token, so every job rebuilt
openshell-test-runner from source. Accept an optional CACHIX_AUTH_TOKEN
secret, pass it to setup-nix, and pass it from Branch E2E, as Branch Checks
and the binary builds already do.

Signed-off-by: Matthew Grossman <mgrossman@nvidia.com>

Author: matthewgrossman

.github/workflows/branch-e2e.yml

@@ -316,6 +316,8 @@ jobs:
     with:
       source-sha: ${{ needs.prepare-integration.outputs.source_sha }}
       integration-inputs-artifact-id: ${{ needs.prepare-integration.outputs.integration_inputs_artifact_id }}
+    secrets:
+      CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
 
   docker-e2e:
     needs: [pr_metadata, build-binaries, build-images]

.github/workflows/e2e-suites.yml

@@ -20,6 +20,10 @@ on:
         description: Immutable ID of the prepared integration inputs artifact
         required: true
         type: string
+    secrets:
+      CACHIX_AUTH_TOKEN:
+        description: Token used to push Nix builds, such as the test runner, to Cachix
+        required: false
 
 permissions:
   actions: read
@@ -66,6 +70,8 @@ jobs:
           exec 3>&-
 
       - uses: ./.github/actions/setup-nix
+        with:
+          cachix-auth-token: ${{ secrets.CACHIX_AUTH_TOKEN }}
 
       - name: Cache tmachine disks
         uses: actions/cache@caa296126883cff596d87d8935842f9db880ef25 # v5.1.0