Clean up run.sh
Clean up run.sh

Clean up the following in run.sh:
* Fix shellcheck warnings.
* Switch to #!/usr/bin/env bash to use the current environment's bash.
* Parse command line flags.
* Add help message.
* Use for loop to dry code up when checking dependencies.
* Send pushd and popd stdout and stderr to /dev/null to declutter script output.
* Check multirust list-toolchains for existing install before overriding.
* Store multirust version in a variable.

Minor:
* Reword a few sentences.

bjtitus/Eve,kidaa/Eve-1,bjtitus/Eve,8l/Eve,pel-daniel/Eve,justintaft/Eve,bjtitus/Eve,bluesnowman/Eve,8l/Eve,pel-daniel/Eve,justintaft/Eve,pel-daniel/Eve,bluesnowman/Eve,8l/Eve,justintaft/Eve,8l/Eve,bluesnowman/Eve,kidaa/Eve-1,kidaa/Eve-1,justintaft/Eve,kidaa/Eve-1,pel-daniel/Eve,8l/Eve,bluesnowman/Eve,bjtitus/Eve