Use `.env` file instead of `.envrc` (#1132)
#1131 shows that `direnv` installation is _most_ of the CI overhead
introduced by #1105.
Instead of using `direnv`, let's just use a simple `.env` file that can
be loaded with `source` or [`direnv`'s `dotenv`
directive](https://direnv.net/man/direnv-stdlib.1.html#codedotenv-ltdotenvpathgtcode).
