[Heartbeat] Only setuid in elastic-agent image (#28577)

Fixes #28572 by only invoking setuid in the elastic-agent container, and
no longer in the heartbeat container. See the linked issue for details.

(cherry picked from commit da2bc89)

dev-tools/packaging/templates/docker/Dockerfile.tmpl

@@ -132,7 +132,6 @@ USER {{ .user }}
 {{- if (and (eq .BeatName "heartbeat") (not (contains .from "ubi-minimal")))  }}
 # Setup synthetics env vars
 ENV ELASTIC_SYNTHETICS_CAPABLE=true
-ENV BEAT_SETUID_AS={{ .user }}
 ENV SUITES_DIR={{ $beatHome }}/suites
 ENV NODE_VERSION=14.17.5
 ENV PATH="$NODE_PATH/node/bin:$PATH"