Change CI script hash-bang from /bin/env to /usr/bin/env

Historically, the `/bin` directory on Unix has a reduced set of
commands used for small mounts while booting the system. As such,
it is common for `/bin` to be missing the `env` command.

For the same historical reasons, `/usr/bin` tends to have most if
not all commands provided to the user environment. Thus,
`/usr/bin/env` is more likely to exist than `/bin/env`. This is
in fact the case for Mac OSX, which is a very widely used version
of *nix.

Thus, it is better to use `#!/usr/bin/env` as the hash-bang in
scripts.

Utilities/CI/reproduce_ci_env.py

@@ -1,4 +1,4 @@
-#!/bin/env python3
+#!/usr/bin/env python3
 
 #=============================================================================
 #