Fix wrong user for installing Synthetics

[rdner]

The current user must be used instead of root.

How to test this PR locally

I copied the testing steps from #40689 and followed them.

• Build heartbeat wolfi image locally with:

cd ./x-pack/heartbeat
PLATFORMS=linux/arm64 PACKAGES=docker mage package

• Create the following monitor configuration (heartbeat.docker.yml):

heartbeat.monitors:
- type: browser
  id: korean-characters
  name: korean_characters_test
  schedule: '@every 1m'
  source:
    inline:
      script: |-
        step("first", async () => {
          await page.goto('https://www.naver.com/');
        });
- type: browser
  id: chinese-characters
  name: chinese_characters_test
  schedule: '@every 1m'
  source:
    inline:
      script: |-
        step("first", async () => {
          await page.goto('https://baike.baidu.com/');
        });
- type: browser
  id: russian-characters
  name: russian_characters_test
  schedule: '@every 1m'
  source:
    inline:
      script: |-
        step("first", async () => {
          await page.goto('https://yandex.ru/');
        });
- type: browser
  id: arabic-characters
  name: arabic_characters_test
  schedule: '@every 1m'
  source:
    inline:
      script: |-
        step("first", async () => {
          await page.goto('https://www.bbc.com/arabic');
        });

• Run the container with this configuration:

#!/bin/bash

docker run -it --rm \
  --name=heartbeat \
  --user=heartbeat \
  --volume="$(pwd)/heartbeat.docker.yml:/usr/share/heartbeat/heartbeat.yml:ro" \
  --cap-add=NET_RAW \
  docker.elastic.co/beats/heartbeat-wolfi:9.0.0 \
  --strict.perms=false \
  -E output.console.pretty=true

• Run and check monitors are executed correctly with status: up.

Related issues

• Relates to [Heartbeat] Add Synthetics to wolfi image #40689
• Relates to Make Wolfi a part of Docker packaging #40524 (the bug was introduced here)

[mergify]

backport-v8.x has been added to help with the transition to the new branch 8.x.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[emilioalvap]

LGTM