Fixes up scripts (except under /linux) to use `#!/usr/bin/env bash`
instead of `#!/bin/bash` or `#!/bin/sh` so that we don't end up with
the ancient version of bash supplied with macOS.
This became urgent with this PR, because of bash-4.xisms in
build-utils.sh, for example on line 572:
```
if [[ -v _builder_params[$e] ]]; then
```
Adds run-xcodebuild function to wrap xcodebuild calls to keep logging
and error handling consistent. Currently used only by the iOS build; the
macOS build has a corresponding execBuildCommand function which does
pretty much the same thing (changing that would belong in a separate PR
I think and low priority.)
The build agent already has the relevant `SENTRY_` environment variables
set in order to complete builds for all projects. Thus, we should
exclude these values from the xcode build. However, we need to ensure
that the `SENTRY_URL` and `SENTRY_ORG` variables are imported into the
xcode environment from the build agent environment.
