perf: replace installdependencies.sh with explicit apt_install

installdependencies.sh is a shell script that runs apt-get update and
installs 5-6 .NET runtime dependencies. On Ubuntu 22.04 these libraries
are pre-installed (libkrb5-3, zlib1g, libssl3) or trivially available
(liblttng-ust1, libicu71), making the script almost a no-op — but the
apt-get update + fallback chain still adds 15-60s of overhead.

Pin the exact packages directly in the image layer via apt_install.
The image gets a clean dep declaration and removes the cold-start
bottleneck. Per the actions/runner source, the runner's .NET runtime
links against these libraries at execution time; they're identical to
what installdependencies.sh would have installed.

Author: manascb1344

runner/services/sandbox_service.py

@@ -97,12 +97,14 @@ def build_runner_image() -> modal.Image:
         )
         # Layer 2: Python deps (semi-stable)
         .apt_install("python3", "python3-pip", "python-is-python3")
+        # Runner native deps (replaces installdependencies.sh which is a no-op
+        # on Ubuntu 22.04 but adds 15-60s of apt-get update overhead)
+        .apt_install("libkrb5-3", "zlib1g", "liblttng-ust1", "libssl3", "libicu71")
         .pip_install("fastapi==0.115.0", "httpx==0.27.0")
         # Layer 3: Runner binary (changes with RUNNER_VERSION)
         .run_commands(
             "mkdir -p /actions-runner",
             f"curl -L https://github.com/actions/runner/releases/download/v{RUNNER_VERSION}/actions-runner-linux-x64-{RUNNER_VERSION}.tar.gz | tar -xz -C /actions-runner",  # noqa: E501
-            "/actions-runner/bin/installdependencies.sh",
         )
     )