fix: attempt to make linux builds faster (#911)

This work aims to make Linux builds faster to make https://github.com/ooni/probe/issues/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`.

GOCACHE/.gitignore

@@ -0,0 +1 @@
+/oonibuild

GOCACHE/README.md

@@ -0,0 +1,8 @@
+# Directory GOCACHE
+
+This directory contains the GOCACHE and GOMODCACHE we use when
+statically compiling Linux binaries using Docker.
+
+If you keep the content of this directory, subsequent builds will be
+faster. You will notice this especially for builds using qemu-user-static
+to build for different architectures.