Replace run-proxy.sh with a binary

[olix0r]

To support #5198, we should use a distroless image for the proxy's runtime (see #6165).

In order to move to a more restricted container image, we can't rely on a shell being present in the runtime container, however the proxy is started by an init script

linkerd2/proxy-identity/run-proxy.sh

Lines 1 to 10 in e784b5b

	#!/usr/bin/env sh
	set -eu
	if [ -z "${LINKERD2_PROXY_IDENTITY_DISABLED:-}" ]; then
	/usr/lib/linkerd/linkerd2-proxy-identity \
	-dir "$LINKERD2_PROXY_IDENTITY_DIR" \
	-name "$LINKERD2_PROXY_IDENTITY_LOCAL_NAME"
	fi
	exec /usr/lib/linkerd/linkerd2-proxy

We should either rewrite this as a small standalone binary or fold this logic directly into the identity helper and then update the proxy's dockerfile (as in https://github.com/linkerd/linkerd2/blob/d8f010d8ceedaba6f0509a3c5539c5a50844f4f4/Dockerfile-proxy) to use a restricted runtime image.