Reducing number of concurrent builds

[achmyr]

Hi,

I've got a problem with cloning the template for my plugin:

• I created Repo from the existing template
• The dependabot created 8 PRs for minor version updates
• I have 9 jobs running, which should take around 15 minutes each.
• Since I approved PRs and merged them, it kicked another 8 jobs,

In total, it took 15*17=255 minutes out of my 2000 min/month limit (for a free tier) and that was even before I started to make any changes.

The change would limit the number of concurrent jobs running at the same time on the main branch saving precious minutes.