chore: speedup replace-env-vars.sh

[erijohnt]

The previous implementation of this script uses find to enumerate all files under the app directory, then reads all of those files in a single while loop to apply sed. This is extremely slow with ~80k files during build time. We don't need to read all these files like that; we can use grep to search for files that we actually need to update and then parallelize the sed command to do it.

I ran shellcheck against the script to make sure I didn't introduce any obvious new footguns and made sure docker-compose -f docker-compose.yml up --build still built the image and ran without issues.

[vercel]

@erijohnt is attempting to deploy a commit to the Plane Team on Vercel.

A member of the Team first needs to authorize it.

[pablohashescobar]

Thank you so much, @erijohnt, for your valuable contribution. We truly appreciate your effort. I'm pleased to inform you that we will be merging this pull request.