`yarn run release` duplicates every Markdown file in our repository

Created by: slimsag

@beyang there seems to be a bug in yarn run release release:publish 3.10.2 where it duplicates every Markdown file in our repo adding a -E suffix to the filename: https://github.com/sourcegraph/sourcegraph/pull/7073/commits/c06fc6ce6634bca649b9a1f43b528b1cecc236a8

This appears to be due to the find -exec sed command it runs, which assumes a Linux sed command is in use. Running brew install gnu-sed and altering the command to use gsed seems to be required:

find . -type f -name '*.md' -exec gsed -i -E 's/sourcegraph\/server:[0-9]+\.[0-9]+\.[0-9]+/sourcegraph\/server:3.10.2/g' '{}' +

Maybe we can just rewrite this as a comby match / replace?