fix: attempt to make linux builds faster

[bassosimone]

This work aims to make Linux builds faster to make ooni/probe#2249 more convenient. Since those builds runs inside Docker, the problem to solve here is to save/restore the Go caches notwithstanding Docker. Because Docker runs as root, we need to modify the build a bit to run as a normal user. Otherwise, we will not be able to save the Go cache using actions/cache@v3. (Other approaches such as using sudo are possible but running the build as an unprivileged user actually looks cleaner, so I chose to do that.) While there, add a .editorconfig.