fix: Backporting 19989 into 3.6

[spiridonov]

What this PR does / why we need it:

Manual backport of #19989 to 3.6 branch as per https://github.com/grafana/support-escalations/issues/20615#issuecomment-4012195758

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

---

Note

Low Risk
Low risk Makefile-only change affecting how promtail is built when PROMTAIL_JOURNAL_ENABLED=true; the main risk is unintended build-flag formatting issues in some environments.

Overview
Adjusts the Makefile promtail build to append the promtail_journal_enabled build tag to the existing Go build flags instead of replacing tags entirely.

When PROMTAIL_JOURNAL_ENABLED=true, promtail and promtail-debug now build with an extra ,promtail_journal_enabled tag added to $(PROMTAIL_GO_FLAGS)/$(PROMTAIL_DEBUG_GO_FLAGS), preserving existing tags like netgo.

^{Written by Cursor Bugbot for commit fdfffaa. This will update automatically on new commits. Configure here.}

[loki-gh-app]

The backport to release-3.6.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-21356-to-release-3.6.x origin/release-3.6.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 0f56890f0536200d1ebb22bcc3488d0e79d9285b

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-21356-to-release-3.6.x
# Create the PR body template
PR_BODY=$(gh pr view 21356 --json body --template 'Backport 0f56890f0536200d1ebb22bcc3488d0e79d9285b from #21356{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title 'fix: Backporting 19989 into 3.6 (backport release-3.6.x)' --body-file - --label 'size/XS' --label 'type/bug' --label 'product-approved' --label 'backport' --base release-3.6.x --milestone release-3.6.x --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-21356-to-release-3.6.x

# Create a pull request where the `base` branch is `release-3.6.x` and the `compare`/`head` branch is `backport-21356-to-release-3.6.x`.

# Remove the local backport branch
git switch main
git branch -D backport-21356-to-release-3.6.x