Use $(...) notation instead of legacy backticked `...`.

https://github.com/koalaman/shellcheck/wiki/SC2006



Signed-off-by: Dan Callahan <danc@element.io>

On NixOS, `bash` isn't under `/bin/bash` but rather in some directory in `$PATH`. Locally, I've been patching those scripts to make them work.

`/usr/bin/env` seems to be the only [portable way](https://unix.stackexchange.com/questions/29608/why-is-it-better-to-use-usr-bin-env-name-instead-of-path-to-name-as-my

) to use binaries from the PATH as interpreters.

Signed-off-by: Quentin Gliech <quentingliech@gmail.com>

* changelog

* reduce circleci config

* plus a handy script

* fix regex