setup_path() writes the user-facing hermes shim with `cat >`, which
follows existing symlinks. Older installs created
`$command_link_dir/hermes` as a symlink to `$HERMES_BIN`
(`venv/bin/hermes`), so re-running install.sh stomped the pip entry
point with a bash shim that exec'd itself in an infinite loop.
`rm -f` the link target before writing so the shim lands at
`$command_link_dir/hermes` and the venv entry point is left intact.
Adds a regression test that reproduces the symlink-stomp end-to-end
(creates the symlink, drives the real shim-write block from setup_path,
asserts the venv pip script body survives and the shim is now a regular
file). Both new assertions fail on origin/main and pass with the fix.
Closes#21454.
