Quote image tag when tagging :latest-with-browser images

[andrewslotin]

What?

Fixes a syntax error in the build pipeline that publishes built Docker images to Docker Hub.

Why?

Current build pipeline is broken due to a syntax error. This prevents CI from pushing the grafana/k6:latest-with-browser images to Docker hub as well as legacy loadimpact/k6

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.

[codecov-commenter]

Codecov Report

Merging #3256 (e7f7492) into master (b7f040b) will decrease coverage by 0.15%.
The diff coverage is n/a.

❗ Current head e7f7492 differs from pull request most recent head 546c391. Consider uploading reports for the commit 546c391 to get more accurate results

@@            Coverage Diff             @@
##           master    #3256      +/-   ##
==========================================
- Coverage   73.22%   73.08%   -0.15%     
==========================================
  Files         259      257       -2     
  Lines       19899    19897       -2     
==========================================
- Hits        14571    14541      -30     
- Misses       4404     4424      +20     
- Partials      924      932       +8     

Flag	Coverage Δ
ubuntu	?
windows	73.08% <ø> (+<0.01%)	⬆️

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see 11 files with indirect coverage changes