Install into $RUNNER_TEMP instead of $HOME (#338)

$HOME can change between composite-action steps on some runners
(e.g. self-hosted ones where actions/checkout temporarily overrides
it), which makes the PATH entry from the setup step point at a
different directory than the install step writes to. The result is
'lychee: command not found' even though install reported success.

$RUNNER_TEMP is guaranteed-writable and stable across steps within
a job, which is exactly what we need here.

Also drops vestigial lychee binary cleanup step.

With $RUNNER_TEMP the install directory is fresh per job, and
'install -t' overwrites existing files by default, so the explicit
'rm -f' step is no longer doing anything useful.

Fixes #337