When the user pins a runtime via the action's `runtime` input, that
runtime has already been installed and put on PATH by the time `pnpm
install` runs. If the project's package.json also declares a different
`devEngines.runtime` with `onFail: download`, pnpm install would fetch
that second runtime and silently swap the active one — defeating the
matrix/explicit pin.

`--no-runtime` (pnpm v11.1.0+) tells pnpm install to skip the runtime
fetch entirely. Appending it whenever the `runtime` input is set
matches user intent: "I told the action which runtime I want, don't
let install second-guess it."

When the input is omitted (runtime fully driven by devEngines.runtime),
`pnpm install` runs unmodified — the devEngines runtime is the only
source of truth and install should honor it.

Adds a `runtime-overrides-devengines` test that pins node@22 in the
action input, declares ^20.0.0 in devEngines.runtime, and asserts node
22 is still active after install.

Bumps test workflow's pnpm pin to 11.1.0 since `--no-runtime` requires it.

Two issues caught by the Windows CI failure on #2:

1. `patchPnpmEnv` (in src/utils) prepended `dest/node_modules/.bin` to PATH
   before spawning `pnpm install` and `pnpm store prune`. On Windows in
   always-standalone mode, `.bin/pnpm.cmd` is an npm-created shim pointing
   at the BOOTSTRAP pnpm (e.g. 11.0.4), not the self-updated target. That
   shim won that PATH race and overrode the self-updated pnpm at
   `$PNPM_HOME/bin`, so `pnpm install --no-runtime` ran on the bootstrap
   and bailed with "Unknown option: 'runtime'". POSIX got lucky because
   `.bin` and `$PNPM_HOME` resolve to the same directory there.

   Removed `patchPnpmEnv` entirely. `spawnSync` now inherits `process.env`,
   whose PATH is already correctly fronted by `$PNPM_HOME/bin` and
   `$PNPM_HOME` via the `addPath` calls in install-pnpm.

2. `--no-runtime` is a pnpm v11.1.0+ flag — passing it to older pnpm
   errors. Added `pnpmSupportsNoRuntime()` which parses `pnpm --version`
   and only appends `--no-runtime` when major.minor >= 11.1. On older
   pnpm with `runtime` input set, the action emits a warning that
   `devEngines.runtime` with `onFail: download` could still shadow the
   explicit runtime.

The whole `src/utils/` module is now empty and has been deleted.