executor: support caller-owned runtimes from action inputs

[longlho]

Browser actions need caller-pinned Node, Chromium, libraries, and fonts at normal Linux paths. Add input-rootfs (or input-rootfs-env for action-generated paths) to expose a declared input directory as the action runtime, without downloading images or mixing in packaged runtimes. Preserve executor-owned devices, procfs, temporary directories, and sandbox policy.

bazel build //... and bazel test //... pass with local execution (78 targets, two test suites). Downstream Linux VM VRT checks pass with this patch plus the independent kernel fix in #33. macOS VM validation was unavailable here.

[longlho]

Closing in favor of perplexityai/rules_web_e2e#31, which relocates declared runtime executables and supplies explicit loader/library/shell paths, removing our need for input-rootfs support.