fix: add CODEX_HOME env var to MCP gateway step for Codex engine workflows (#27457)

* fix: add CODEX_HOME env var to MCP gateway step for Codex engine workflows

The MCP gateway setup step needs CODEX_HOME to be available as an
environment variable so that the Codex config sync (copying the
converted MCP config into CODEX_HOME) can reference it. Without this,
mkdir -p "${CODEX_HOME}" expands to an empty string and fails.

Adds CODEX_HOME=/tmp/gh-aw/mcp-config to collectMCPEnvironmentVariables
only when the engine is codex, matching the value already set on the
agent execution step in codex_engine.go.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chore: start addressing PR review feedback

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/27e0a550-30d5-4846-8d4e-03d3761d5659

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: 3 people

.github/aw/actions-lock.json

@@ -83,6 +83,11 @@
       "version": "v5.2.0",
       "sha": "be666c2fcd27ec809703dec50e508c2fdc7f6654"
     },
+    "actions/setup-node@v6.3.0": {
+      "repo": "actions/setup-node",
+      "version": "v6.3.0",
+      "sha": "53b83947a5a98c8d113130e565377fae1a50d02f"
+    },
     "actions/setup-node@v6.4.0": {
       "repo": "actions/setup-node",
       "version": "v6.4.0",