`run-vitest-coverage.mjs` reused `process.env.npm_execpath` with
`process.execPath` (node) for a fast path that skips the pnpm wrapper.
That assumes `npm_execpath` is a JS file, but standalone pnpm installs
ship a native Mach-O / ELF binary, so node tries to load the binary as
a CJS module and crashes with `SyntaxError: Invalid or unexpected
token`, taking the pre-push hook down with it.
Only take the fast path when the path actually looks like a JS module
(`.js`/`.mjs`/`.cjs`); otherwise fall back to invoking `pnpm` from
PATH, which is what the script already does when `npm_execpath` is
unset.
